Problem is, he doesn't even remember its name.

What he remembers is the following:
-It was a platform game
-Main character was a pirate... or so my pal remembers
-Some of the levels included swimming
-There were several different weapons to use
-It has been released for Amiga, he thinks it also had a PC version

Hmm... That's not very much info :unsure:. I'll take a look and see if I can find anything...

Forgot one thing. This game has been - according to him - released for Amiga, altough he thinks there also was a PC version.

Wild guess: Super Wonderboy in Monsterland? (not pirate)

I had ATLEAST 15 different platform-games with a char. that used different weapons and where you had swimmin included, to my Amiga 500 and 600 :(

I know this topic is kinda old, but I dunno if fifth has found what he's looking for yet, so here goes - I know this game isn't that old but I dunno if your friend was talking about an oldie or not - could it be Pirates - The Legend Of Black Kat, by Westwood? Probably not, but...meh.

Still looking?
 

Okey, I'm veeeery late with this, and I hope your friend have already found the game - but if not, I'm sure the game he's looking for is Traps n' Treasures made by Starbyte Software in 1993. It's actually my favorite Amiga game. Info and screenshots are found here.

It's traps'n'treasures for sure. We recently spoke about that game on domestic "old-games" forum. Unfortunately, I don't think that any other version exists. Boy I wish I had amiga when I was a kid..