actually they don’t really function as “boats”, they’re more like very mobile underwater houses, as ocean environments being less “grounded” than their terrestrial counterparts (and also the dangerous levels of persecution the United Archipelago puts on merfolk) means that merfolk tend to live highly nomadic lifestyles where there’s no place or time to really settle down anywhere. despite this they still gather together in pretty large communities like regular humans do, with multiple of these “houseships” congregating like a school of fish into a “village” “town” or even “city”.

there’s multiple wildly different types of models for houseships in all shapes and colors and sizes, for various biomes and pressure levels and whatnot, with most of them being for purely underwater use, though i imagine this specific one i’ve drawn being a “mixed under/overwater” model used by merfolk performing tasks outside of the water (usually as part of an outlanders society), due to the larger top fin seeming more efficient with surface wind than with underwater currents. houseships tend to be pretty lightweight even with decent cargo (underwater physics is handy) and are made with a kind of flexible but sturdy water-resistant wood that’s fairly easy to fix and mend and is also found from certain underwater trees (which are actually mutated from regular land trees, which ended up sinking completely underwater due to the great flood. it’s kinda weird). also the fish-like design is a result of biomimicry intended for camouflage (don’t want to spook or aggravate larger fish) (don’t want to end up turning into fish filet by the nearest stationed mermaid hunter squadron) (also it’s hydrodynamic as fuuuuuuuuuck)

the average houseboat can house up to 5-7 human-sized merfolk pretty comfortably; the smallest ones usually go only up to two people, because one of them has to be in the cockpit controlling the boat whenever it’s needed while the other manages the boat’s resources and other stuff (or just vibes); meanwhile the biggest ones are incredible feats of underwater architecture nearly the size of entire cruise liners (typically used for lavish balls and festivals and other events that warrant larger indoor space), though due to the inconvenience of their size they tend to actually be at least a couple of smaller ships “trenchcoating” together as a much larger unit, which can split off in case of an emergency.

not entirely sure of the full mechanisms behind these ships, but i imagine poles levers and pullies used to open/close/move different sails, and also to rapidly pump water in and out through specialized water-cycling tanks/tubes for various directions of locomotion (maybe that part could start functioning through something similar to a fish’s gill’s…?). building and fixing the intricate internal mechanisms of these vehicles is an important and worthy career to have guaranteed to pay you some good pearls. (along with building the ships themselves. and refurbishing the interiors. and giving them paintjobs. and even salvaging parts of them from wrecks. we got a whole very loose industry going on here)