20w50 all the way, way more so for your hot climate.
15/40 acceptable but would prefer 25/40 is you were to go down to a 40.
10w30 to me would be insanity and I think all you would need to do is try it out, then run the boat for an hour or two at 3000+ RPM, slow down and listen to your valves clap.
Most marine engines have 170 degree thermostats in them so it doesnt matter too much as far as water temperature that the boat is running, unless you have closed cooling, then you may have a 190 thermostat..
Oil temperatures do not correlate to water and engine temperature. An engine with a water/coolant temperature of 170 degrees will see oil temperatures well over 200 degrees, there is nothing cooling the oil and your engine compartment is pretty much closed.
This isnt an automobile with 4 smooth rolling wheels, this is a log (boat) being pushed through water with a lot of engine and bearing pressure which greatly adds to the temperature of the oil.
(just my thoughts)
Ps. I always followed the manufacturer which was 25/40 in my mercruisers and now with a Volvopenta engine the book also states 20/50 as an option that I choose to use.
Actually currently using 15w50 Mystik JT8 semi syn this year in our new to us Chaparral.