Swimming

With running / cycling, I can get into a state where I want to die quite easily, but safe in the knowledge that my lungs might burst and my chest explode, but I can't do that in the pool without significant drowning risk - My stroke rate can't keep up with my need to breathe faster and deeper.

What intervals are you trying to do? Try starting from a low base; say one fast length every six or eight and increase the frequency of the fast lengths as you get more comfortable.