An HIIT drill I'd do in the pool is something I called 'minute intervals'. Basically sprint a length at maxuimum effort at 0:00 and rest at the other end until 1:00, then sprint back, rest until 2:00, and continue for the designated time, usually 20 minutes.
It's rather difficult towards the end because your rest times decrease as fatigue kicks in, plus you get some perculiar looks doing this whilst everyone else generally plods along up and down. You may find you're continually overtaking and being overtaken by the same person numerous times!!