To get back to the original question, if you're simply bolting timber parts
together and it's not an offshore powerboat, loctite, lock washers and nylocks
are not needed. Friction does the job.

Adam wrote:

I'm going to install floor stiffeners (2"x4" cedar sealed 3x by S1 - floors
are 1/2" marine plywood sealed as well) using machine SS bolts 3/8" 3", 4" &
5" long.
Should I use any of Lctie for the nuts?
Thanks in advance
Adam