I usually go for some combo of the following:
1. Before I put on my wetsuit, a casual 5 min jog with some pickups towards the end. Nothing too strenuous tho.
2. Break out the pull chords and do 3 rounds of 20 pulls and 15 squats. Pulling hard-ish, but not blowing up or anything.
3. If they let me get in the water, I do 7-9 minutes of swimming. Mostly easy, but I throw in 2-3 rounds of 20 hard strokes (usually towards the end) to warm up the lactate system. To be fair, this is how I usually warm up for my long swim so it’s not a big change.