In my opinion, it depends upon the
boat, boat construction, sea valve location within the boat, the boat's future intended usage, the boat's
electrical complexity, and where the boat it moored and that
environment, etc. There is no one one "best" sea cock.
Personally, & assuming the boat is FRP, I prefer the traditional flanged
bronze coned sea cock (several manufactures) positioned in the boat so that it is easily accessible & serviceable. When installed via monel bolts, bolted to the
hull skin (w or w/o the through hull) properly bedded to the
hull, these will last almost for ever. I am also of the opinion that they are the strongest impact resistant valve
installation & most forgiving vis-a-vis internal abrasion as they can be serviced.
However, as the above boat becomes more electrically complex (Ie. bonding systems &/or moves into a
mooring environment where the is a probability of stray
current fields within the area/mooring basin, (especially if the boat has a full bonding system) I personally believe the advantage shifts to one of the dielectric composite valve systems. As the suspended abrasives (ie. sand, etc.) in the water increases, I am of the opinion that the advantages begin to shift away. If the boat is wet moored in an area subject to freezing and the sea valve is one that must remain open (ie:
cockpit drain), the advantage shifts back to the "traditional"
bronze valve.
While I am not a fan of the ball valve mounted on solely on a threaded through hull (assuming proper thread compatibility & thread engagement), I am of the opinion that the advantage shift to them when the valve is located in a restricted area where it is not easily serviced (as is so common on most production boats) and there is nothing in that space that could shift & place excessive loads on the valve. The advantage of the ball valve is that it requires little
maintenance & has a compact size so that it is easy to remove & replace should it develop problems.
All valve have their merits & issues. All, like everything on a boat, require periodic
maintenance &
inspection.
Don't get me started on the above water line valves. IMHO, the plastic though-hull above the waterline without some method or valve to close the hull penetration when the through hull fails is an problem waiting to happen.
Just my $.02.