i agree with steve that u should pay attention to the inside.
1. for the steel vessel, if the do the full blasting prior to painting,
then the result is superior. dont forget the most important thing-
blasting. after blasting u should paint the marine primer then
undercoat and then coal tar expoxy and then to avoid marine growth, u
should have the antifouling painted.
2. Usually boat owner will dock their vessel 2 to 3 years for
inspection of the underwater hull and do some repair that have to be
done during docking like inspection/repair of the seawater intake valve
or strainer. u cant repair this item when the vessel is in the water.
3. coal tar expoxy in my experience is a very good paint and though
there are many new paint at the market now but for me, if the price is
reasonable, u still can use this paint.
4. if your vessel is stationary there, not moving very often where u
will not scratch your underwater hull by those floating wood or
rubbish, then the coal tar expoxy will last for many years but i am not
sure it will last 30 years. if u do the blasting and follow the paint
scheme, then the underwater hull really may last for many years.
5. In my country many boat owners like to use coal tar because it is
cheaper then other new paint system.
hope this will help.