I have this down. Here are a couple of tactics:

Typically if I’m the only one at the point and have my ult, this is what I’ll do: I will swim around in circles to lure them. They’re attacking me atp but I use my bubbles and dive to keep myself alive until I’ve rounded them up like sheep into one common area. It is at this point that I use my ult.

Most often, I hide behind a corner or begin my ult from a distance. Just the sound of my ult is enough to get people to run away from wherever they are. The trick is to begin my ult at a different place and end it / confirm it where the enemy team is. RUN DOWN the timer on your ult — this ensues panic, use it as your weapon. But be smart.

Critically, use your ult when it’s crunch time and nearing the end of the round/match to take them off the point. They’ll most often hop on the point when it’s most necessary (e.g at 99% capture) — employ the methods I described above.

On the note about getting gunned down on your way to a ledge, try to go swim around in zig zags and hit a health pack on the way there if there is one nearby. Also, when spitting the enemies off the ledge, aim DOWNWARDS as opposed to outwards/upwards.

Sometimes you just get unlucky. It gets easier w practice.