dtgreene: No, it's not unplayable, but there are a few caveats:
* The whirlpool moves in real time, according to the game's framerate, and on a modern system the framerate is *really* high, so you might want to slow the game down. (If using DOSBox and the whirlpool is moving too fast, reduce the number of cycles.)
* Enemies are scarce on the world map, to the point where you'll want to get your early combats in either in town (watch out for guards!) and dungeons (but don't go too deep until you're ready!)
* Scarce overworld enemies means that ships are rarer than they should be, but at least there's an exploit that lets you work around it (though using that exploit involves the death of a party member, which triggers an autosave).

So, yes, the game is playable without the upgrade patch.

(With that said, the situation is still better than Ultima 2, where trying to play the game on a modern system without slowing it *way* down would result in a division by zero error and the game not starting.)