Thetis blessing is a sick joke, and in MP I found exactly one use for it.
I cast it and let R'lyeh think I was using it to survive under water (I wasn't, I had enough WB items to keep everyone alive). He wasted some huge number of astral pearls bringing it down after I cast it at minimum strength
.
That was a pretty unusual circumstance, however. The spell is basically a trap for the unwary.