I much prefer them to the other kinds of elemental, as they have far more character than 'a big living wave' and feel more like living things. Let's make waves...
Art
I have mixed feelings on this piece. The face is amazingly characterful and the face slightly looking over the shoulder just screams low cunning - I can imagine being ripped off by this corpulent cod-man. It manages the rare feat of having personality and character yet looking totally alien. There are some good little characterful designs in the clothing which are the kind of tiny hooks which can make a race memorable to players: are those big shoulders an example of Marid power-dressing? What's that fish on his belt for? I think a key inspiration here is the Vodyanoi of the Bas-Lag universe, which I can always get behind.
However, I hate the 'below the waist I am water/air/fire/some rocks' conceit in elemental design, and here is looks bizarre and amateurish. Because the Marid is a chubby fellow it looks even more incongruous. There's also the lack of thought - why does this creature wear obviously not waterproof clothing made from cloth? Wouldn't everything in that bag get soaked in water? Why is he wearing a belt when he doesn't have legs, never mind trousers? The fluff even claims they wear pantaloons...
Purpose and Tactics
The Marid, being an intelligent and powerful Chaotic Neutral creature, could easily be a quest-giver, patron, rival or contact of the player-characters, especially in an aquatic or maritime campaign. I'd love to run one as a rival to a party, swimming around on some half-sunk barge with slaves in tow.
Should you use one as an antagonist, they are not the most interesting battlers, working best as a boss supporting a group of damaging melee combatants like Sauhagin, Kuo-Toa, Chuul, Sharks or Merfolk. The most interesting mise-en-scene for such a battle would be a rocky coastline, island chain, coral reef, shipwreck, cliff-face or other place where there is an interplay of air, water and land, foregrounding the advantages and tactics of the Marid. His spells are fairly useless in an actual fight, and probably frustrating to player-characters if he is an antagonist: he can use Invisibility, Plane Shift or Gaseous Form to easily escape. He also carries a load of spells clearly intended to be helpful to a friendly party: Water-Walk, Water Breathing, Purify Food and Water, Tongues...The only spells that will see much use in a scrap are Conjure Elemental (note the one minute casting time though), Fog Cloud, Invisibility and Control Water. Aside from buffing sneaky minions with Invisibility or Fog Cloud, most actions spent on casting are a waste.
This reduces our Marid to a rather shallow pool of combat options. He has a Trident multiattack which deals reasonable if unimpressive damage - you should always use it two-handed in melee as the Marid has no shield and does not require an arcane focus etc for its innate spellcasting. The Water Jet is a pretty tidy supportive option, potentially pushing back foes and knocking them prone so your minions can smack them with advantage.
The Marid's other abilities revolve around being really, really time-consuming to kill. Ignoring the previously mentioned ability to just nope out of a combat with your players with Plane Shift or Gaseous Form, it also has a very fast swim and fly speed, a big pile of hit points and a few resistances to shore it up even more. I personally don't find battles of attrition all that fun, but a chase scene or combat where killing the Marid is not the direct goal could still be very fun - rescuing people from a sinking ship, hunting a great white whale or escaping a flood perhaps.
Fluff
The Marid gets a pretty big write-up for a Z-list monster which very few people will use and which can't be summoned with Conjure Elemental. Much of this info would make for a very memorable recurring NPC: they're egotistical and obsessed with self-aggrandising titles (never introduce a Bob the Marid when you can introduce Sovereign of Saltwater, Sultan of the Seven Seas, Most Resplendent Emperor of Dew and Rain and Damp, Pearl of the Oceans, His Supreme Magnificence the Imperator Bob the Marid ...) They collect slaves but get a pass on being kicked down into chaotic evil because they're mostly just baubles - more of an entourage for this Z lister than someone who lowers themselves to work. They also possess an almost bardic focus on stories, tales and legends: something which always makes an NPC interesting. I'd love to hear more of their coral fortresses, but I find its hard to tow the line between the tone of good underwater fantasy and silly My Little Mermaid-esque Disney underwater fantasy.
It's also full of rich tidbits for plot-hooks and adventure ideas: Marids politicking with wizards, Marids seeking out stories, Marids trapped in conch-shells, Marids kidnapping rock stars for their underwater courts....
Plot Hooks
Thuuloso the Ever-Wise, Countess of Crashing Tides can scarcely contain her rage. She shatters boat-hulls and throws up storms to lash at the shore in impotent fury. A cunning magi has stolen her beloved wife and secreted her in an old wine bottle to blackmail Thuuloso into obedience. She would like some mercenaries to walk their 'feet' over the dry lands to find this wine bottle and humble the braggart who would tear her from her soul-mate.
Sulis, Master of the Maberonian Sea, swans about on his great under-water flotilla across the sea, served by his chained mer-men and propelled by his prized hunting-sharks. Should it take his fancy he will drift underneath a mortal ship: some heavy-hulled merchant vessel groaning with amber, antler and fur, or some sharp blade-prowed warship with full complement of ballistae and and braggadocio - or even some low-decked fishing-sloop where a lone and aging man feeds his ever-growing family. From each Sulis will exact a tribute: a fine pearl, or strange delicacy from the land-dwellers (this small brown orb contains the heart of a great oak tree) or spun tale or joke or limerick or service - it matters not the tribute, for Sulis is as capricious as the sea itself.
Throughout the Gilded Sea there is an old legend about the island of Palette's End. Its jagged, romantic visage has inspired countless artists to greatness and yet....there are the disappearances. The losses. Some say artists, ever flighty and prone to melancholy, might have thrown themselves in the sea. Others whisper that the starving artists, penniless, fell victim to debt-collectors. Peasants gossip of the Witch of the Waves who plucks them like o'er-ripe fruit and drags them to her undersea court. But the fact remains - whenever the truly great stay to long on that strange, water-wracked coast, they disappear...
Verdict: A reasonably interesting creature concept let down by a stat-block which doesn't give much for the players to chew on in combat.