If you have your boat's original documentation, see if your engine mfg. included a viscosity-temperature chart. My '96 Merc. 4.3 manual pushes Merc's 25W40, but also states 20W <32 degrees F> 30W <50 degrees F> 40W over 50F. Merc also states 20W-40 or 20W50 are acceptable. I rarely boat under 50F. But I will start it on muffs at colder temps for winterization fogging, oil and drive lube changes. I use synthetics, whitch have better cold start lubrication than conventional engine oil.
So XW-40 is the way to go for summer boating. >50W may be a little too heavy. I've tried to find a 25W-40 full syn, with no luck. Valvoline and Royal Purple offer 40W syns, but they are pricey. The good news is they are a fairly safe bet for two year change intervals, assuming you don't rack up too many hrs.
The average family-mobile probably averages 45-50 mph max in a year. For a 6000 mile oil change that's 120-133 hrs/year. That is a lot of use in a boat year. Condensation, particularly in winter layover is an issue. But I'm good with 2 yr. syn oil changes on my <20hrs/yr boat.