J Ganz said:
SS screws and nylon locknuts

Not sure that's so great an answer, they will have to be very skinny
to fit thru a cotter pin hole and very likely to have the threads
banged up. And the heads & nuts will need room to fit against the body
of the turnbuckle so it will be bulkier, likelier to trap salt water
in crevices too.

Dave wrote:
I assume you mean either machine screws or bolts.

yeah with Whitworth threads

Frankly I don't see what the problem is with cotter pins, if you have
the right tool for installing/removign them, they are very easy to
work. It's not so difficult even with just a pair of cheap needle-nose
pliers.

One thing to be wary of is re-using cotter pins. They tend to break
from being bent/straightened and fall out.

Fresh Breezes- Doug King