My opinion, for what it's worth...

I wouldn't use Glaser Safety Slugs (I'm guessing they're falling under CorBon now).

To quickly and reliably stop a threat, a self defense round needs to penetrate deep enough to disrupt internal organs, nerves, blood vessels, etc. Safety Slugs do not penetrate deeply enough. They may work, but if it does it's more likely a result of the attacker giving up than physical effect, i.e. a psychological stop vs. a physiological stop.

Even if you did accept the risk of using a less effective round than a modern HP, there are other strikes against them. First, they're really expensive. Function testing is important to make sure the ammo is reliable in your gun(s). While revolvers don't require as many rounds as a semi-auto to test reliability, they do require some test firing. You also need to know how the rounds will shoot, which brings up the second issue: POA/POI. Safety slugs are much lighter and faster than conventional bullets. As a result, they may impact someplace other than where you were aiming at. At typical self defense distances, particularly inside a boat, it may not matter, but it is something to consider.

Personally, if I were in your position, I would just choose a good, modern JHP (HST and Gold Dots are hard to beat) and then try as best as I could to anticipate lines of fire to minimize the risk to the boat and bystanders.

For comparison, the last I heard U.S. Air Marshals carry Sig P229s loaded with .357Sig JHP rounds, but they train to avoid hitting important aircraft systems, like electrical and hydraulic systems, by preplanning their lines of fire whenever possible.