What does the manufacturer recommended oil chart look like? Depending on what they suggest for viscosity, I'd probably run either a 15W-40 or 10W-30 HDEO (like the Rotella mentioned above). Likely no need for synthetic in your application and boats don't typically operate in cold enough conditions to need a 5W-40 or other synthetic-only grade. Modern multi-viscosity oils are a lot better than they used to be, so unless you have an engine known for extreme issues with shearing oil (like a 2 stroke Detroit), there's rarely a reason to run straight weight. I'd rather have the lower startup wear from a multi-grade.
Personally, I tend to default to HDEO type oils as my default oil for any engine (gas or diesel) unless the viscosity requirements for the engine dictate something else or there's a specific spec that's required (and no information available to indicate that something else will show good oil analysis, etc. if substituted).
I've mostly been using Delo rather than Rotella for the last few years because at some point, it became cheaper locally. Before that, everything got Rotella. Currently I've got 15W-40 in the boat (syn blend this time around as it was cheaper than the dino version), 5W-40 in 2 of the cars and the snowblower and their 10W-30 in the lawnmower. The 3rd car requires thinner oil, so it gets different stuff.
