3D printing a boat hull I am wondering if it would it be feasible (at an affordable price) to 3D print a boat hull (small dimensions, maybe something like 60x40x20 cm).  

I am mostly concerned about:  


durability (against salty water, UV rays, extreme temperatures
(under the sun or in a cold ocean)
strength (the material should be able to resist some chocs and maybe a little bit of pressure if a wave was to smash on it).
waterproofness


Those characteristics should last during extended periods of time in water (at least several months, maybe more, about a year or two).  

Is there any easily accessible 3D printing material that would match those characteristics?
You will really need to specify your constraints better because the short answer is yes, what you describe is entirely possible, but without knowing whether you are limited to a particular budget, process, or aesthetic, it's not a particularly useful answer.

Some machines (ex. Stratasys Connex 1000) will print models up to 1m in length, so sure, you could print an entire hull with the dimensions you specify.

Pros:


Monohull construction
Excellent surface finish
Many resin options are UV and salt water resistant with decent enough durability


Cons:


Ridiculously expensive machine with decently expensive resins
It will waste plenty of support material in printing (which means added cost too)
Not really easily accessible, but some design studios will have them and will print things for you, for a cost


Other machines (ex. Ultimaker 2 Extended) will print models up to 30cm along the vertical axis. It would require some assembly in the end, but you could segment your build and get a boat hull in the end.

Pros:


Easily accessible
Fairly low cost (Maybe under \$1000 for the machine vs nearly \$1M and many filaments cost 1/10th that of polyjet resins or sintering powders)
The materials themselves can be UV resistant and salt water resistant


Cons:


Joinery and seams create passageways for water ingress, so you'd need secondary sealant
The FDM process itself isn't always watertight, so you'd need sealant anyway
Low interlayer adhesion limits the tensile strength along one axis and the shear strength in one plane, so you'd either need composite hull panels with varying print orientations (in which case, just do a composite layup instead) or a fairly careful analysis of principle hydrodynamic stresses


There's significantly more to the discussion as well, but without really understanding your design constraints, it's difficult to give any concrete advice.