Combat on a ship at sea... how?

[ForceUser]

I'm having a problem with nautical encounters... how the heck do the monsters get aboard the ship to fight? I sent some Sahuagin (sp) to attack the ship, and the players reply "How'd they climb the ship without us noticing?" So the next time, I allow a spot check from a crew member, and they take out the creatures like fish in a barrel. I plan on having lots of ocean travel, and I want my encounters to be fun, not a pain.

I ran a similar encounter once. My sahuagin snuck onboard during cover of night, while everyone but a few sailors were on watch. If the PCs are on watch, chances are that they would have noticed something was amiss, because PCs are cool like that. But my group didn't; all the PCs were alseep. What resulted was a pitched battle in cramped quarters. Lots of fun. Tons of flanks for both sides. Casters were hating life.