My 454 with about 800hrs holds 40psi at idle, and 60psi+ at any engine speed above idle. Your numbers do seem a little low, but not horrible. You have already verified your oil pressure since you have a gauge, but swapping it out with a known good gauge won't hurt to double check pressure readings. There is a very old automotive rule of thumb, that dates back to when these engines were manufactured. 10psi of oil pressure for every 1000rpms being acceptable. So at 5k rpms you should have 50psi, 3k rpms 30psi and so on. If you choose to use this age old rule of thumb, then you are still with in acceptable limits, albeit on the lower end of the spectrum.
In all reality, with the thicker oil, and still showing lowish oil pressure, your engine is probably getting worn out and will be in need of a rebuild sometime in the distant future. It's quite possible that it will run just the way it is for a very long time, a lot of it depends on how you use your boat. If it were me, i wouldn't lose much sleep over it because there isn't much you can do to fix oil pressure with the engine in the boat. Keep an eye on it and enjoy it as you would normally. Rebuild or replace when the time comes.
1988 Supra Sun Sport 454 PCM
1972 Olds Cutlass 455
1998 BMW 750il
1996 BMW M3 coupe
1995 Toyota Tacoma 4x4