When all else fails read the owners manuel. You can either listen to people on here, listen to the oil manufactures or listen to the engineers who designed the engine. These people have no motivation to sell you anything and they know the most about the engine.
My owners manuel for a 91 Prostar 190. It has a 351 indmar, much like yours. It says 30w.
That's it. No 10w-30 or 5w-30. Just straight 30w oil.
Unless you are using your boat in temps below freezing you don't need the 10w-30.
In your lawn mower, motorcycle etc..... use straight 30 weight unless you will be using in temps below freezing. In temps that get down in the teens and 0's, use 5w.
Do a google for oil vescosity. It is very interesting. I did it last year and learned a bunch.
Makes feel much more educated. It made me understand what kind of oil to use instead of just guessing.