Maybe they are below the water line when full
Uh they were still full when they were attacked, they hadn't off loaded anything yet.
The point of attaching the limpet above the waterline is to be able to ignite the streaming fuel. It’s tricky to do. If the hole is below the waterline, it’s just pollution. The mine or mines, planted above the waterline, must accomplish two things:
One, breach the hull and get the fuel flowing through the air into the sea. T
wo, ignite the fumes to ignite the stream of fuel and start a fire that won’t go out.
This is a pretty sophisticated demolition trick, with a good likelihood of failure.
To do it successfully at all would require practicing on a dummy target in port. This was not a group of yahoos. I’m guessing IRGC Quds Force frogmen.
In my old training against big fuel tanks on land, it was always a one-two punch. One demo charge to open up the tank, and separate thermite charges to ignite the spreading fuel.