But how often do you go underwater, and money comes cheap in the game, most of the time I had way more than I need, and I did fine at 300 carry capacity
But you clearly have your own style of play that works well in those conditions. And that's cool for your play style. But I am a thief and I will not be happy with lots of money, I like lots of items. So, ever increasing carry capacity is important to my play style. Currently at level 45 on my thief/smith I have a carry capacity just over 500. And I actually do quite a bit of swimming. There are lots of sunken treasures throughout Skyrim, and while you don't necessarily need to be underwater that long it certainly doesn't hurt to have unlimited time down there. I don't really care about prices that much but it's a nice little bonus. Since I'm playing a complete kleptomaniac carry capacity is the more important thing for me.
Also, something the wiki entry on this mask points out (that I have not exploited personally) is that you can use water to avoid a fight if you want to. So, this mask would help in that regard also.