I'm writing a fantasy story with a race of intelligent dragons who can increase the growth rate of/manipulate plants. If we pretend this is possible, what methods could these creatures utilize to keep the ships alive despite the lack of available soil nutrients and constant exposure to high salinity water? consider also they would be using plants that can tolerate salt to some degree, such as mangroves and sea grass, kelp, ect.
Plants live in the sea, even small plants floating in the sea, so there's no real barrier. Your tree-boats aren't different than a mangrove with the roots on the insides. Grow a hull on land, and solar (plant) power desalinates water. The plant skin can be cactus-like but for the reverse reasons. Waterproofing keeps out salt water instead of keeping water in. The "anchor" (a modified root with a weight on the end) can sample the bottom and net soil to add to the supply if it has needed nutrients. Leaves and branches supply a self-renewing set of sails that can be coaxed to deploy and retract like leaves do in real life in response to varied stimuli.
Carry soil in the hull, or a premade enriched nutrient mix. Go fishing to provide supplement nutrients for the plants (or eat the fish, and use fecal material/bones as compost). Native Americans would bury caught fish in a mound with corn to provide nutrients for the corn to grow. Freshwater fish absorb salt, saltwater ones excrete salt, so there would be no significant difference. The ship can even grow it's own bait (fruits) and could even do the fishing Venus flytrap-style (grabbers or modified root-hooks). The plants can grow vitamin and micronutrient-rich berries to fill in the nutrition of the crew (no scurvey here!) Magic druidic plant charming fills in the gaps.
If the sea-foresters can coax the boat into growing into whatever shapes they want, this is a self-healing adaptive vessel that will help feed the crew while they work to get the tree the things it needs. Other than an occasional stop on land to replenish easier supplies (dirt and fresh water would still be easier if you can get it on land) you could have a whole culture that would hardly ever need to touch land. Smaller boats used to row up to shore can be immature versions of the plant that are later shaped into full ships, but otherwise carried aboard or towed.
You could have nesting interconnected symbiotic relationships here, where an insect that might have a close relationship with trees (like termites) have evolved to shape and manipulate the trees and wood. What was once termites growing their own nests has become a three-way symbiosis. Termites are capable of some extremely sophisticated nest engineering, and in this scenario, they are also guided by intelligent beings. Your ship's captain is in a deep interconnected relationship with the termite queen (this could be mind control, or it may be physical, like a symbiote that is biologically joined to the captain). The captain of the ship can consciously control the termites, who lay down hormone trails the plants grow along. The termites can regurgitate wood pulp to directly reshape the ship, digest the parts you want to get rid of, and so on.
The relationship would be very close, and a good captain would be amazing. A bad captain would leave the ship out of control and possibly kill everyone. The death of the queen or the captain would be a very traumatic event for the entire ship, with much reshuffling (possibly even physical reshuffling) until the new relationship was worked out.
Such termites could serve a variety of critical functions. The termites might be useful as food, or they might produce a secondary food stuff useable by the crew. They could manage the compost, feeding it to the tree. The tree might produce food directly to feed the termites so they don't need to eat their homes to live. The reproductive flying caste could pollenate the trees, and they could even be used as scouts or to communication between ships. They could root out worms trying to eat the wood of the tree. Every nifty function you can think of insects doing could be carried out by your termites. Who knows? Bioluminescent lighting? Maybe they even work in defense of the ship, repelling enemies with poison stingers. Why not?
They could use a 3 shells system:
- the inner shell would be "dead" material, like wood,
- the middle layer would be nutrient carrying material, sort of a compost,
- the outer layer would be made of a lignin producing, genetically engineered variant of kelp or posidonia oceanica, which would use the nutrients from the middle layer to grow.
They would periodically "shave" the outside of the outer layer to keep a decently hydrodynamic profile and to harvest material for repairing, improving the inner layer, while the outer layer would be taken care by its growth.