Oh I realized then that it was an insane action- if the to-hit roll went badly, he could have ended up floating off into wildspace. Even with a hit, he could have ended up, as I mentioned, splattered on the side of the hull and taken serious if not mortal damage. The chances of ending up on deck, or close enough to grab rigging (or maybe hitting a mast) were slim, and I could have asked for a Dex check to not end up orbiting the ship's gravity plane.
But none of those outcomes would have served the needs of the game. I'd have a dead player now sitting out the rest of the session until I could introduce a new PC for them. I was already certain that the players were going to win the engagement, as it was just a stepping stone to the next confrontation (I was running "Letters of Marque"). Normally, I do let the dice decide; certainly, if the player's Dwarf had died, he couldn't blame me, it was his own doing, which, in my mind, if someone is to have a character die, that's a reasonable circumstance.
But in this instance, I didn't see how it would make the game any more fun (maybe as a cautionary tale, or for people to recall "that one time Fitz committed suicide by catapult"). So instead, I gave them "that one time Fitz launched himself at an enemy ship!".
Ironically, the Dwarf didn't last until the end of the campaign; he eventually decided to go out in a blaze of glory- he'd acquired a powerful magical gauntlet that made an explosion when he punched people with it (thus with both him and the target being in the blast) and eventually decided to use it to take out a Hammership (and himself) by detonating the ship's magazine (it was stocking bombards).
Another death I can say I was satisfied with, as he was able to leave an impact (heh) on the game.