The concept without the unnecessary targeting thing has existed since GDS2. I think it was called like "dreamwalk" there. This has also been heavily featured in the "Dreamscape" custom set. I think the usual suggestions are to have this in blue-red, but blue-black isn't that much of a stretch either.
Dreamwalk (This creature can't be blocked by untapped creatures. Tapped creatures can block it.)
I don't get how "only tapped creatures can block this" captures the flavor swimming tbh.
I don't get how "only tapped creatures can block this" captures the flavor swimming tbh.