Not a bad idea, but they don't seem to want to make the attribute list too long. Too long a list and it gets harder to search (for example, by overlooking the right attribute and choosing a similar one instead). Why not just use both attributes for a cache in between? Then in the description explain that it's a bit more than swimming, but full scuba gear isn't needed.
Using both attributes would filter out caches for someone that excluded "scuba" caches because they weren't certified. I'd just use that swimming attribute and make it clear in the description that a diving mask and/or snorkle is required.
Or use the swimming and "special tools required" attribute. Specify the diving mask and/or snorkel as the tool required.
Have no problem with that I just thought is would be cool with a specific attribute for this. And by the way many of the existing attributes could be replaced with just "the special tool" attribute and a little text in the description. But that would make it hard to search for.
But as I suspected it is hard to get anything new into GC.
Thanks anyway