IIRC, you simply look in the direction you wish to swim, and move forwards.
I believe the issue you are having lies elsewhere, in regards to this quest.
The first thing you want to be doing is bolstering your radiation resistance. I watched a level 20 friend complete this, and the first thing he did was use some Rad-X. It significantly decreased the ammount of radiation he absorbed. You could also go for more permanent protection, such as lead armor, or a reinforced Vault 111 jumpsuit.
The second thing you want to be doing is paying attention to those bubbles. By identifying the bubbles forming at the top, you can identify where the leaks are, before having to dive in. Furthermore, the bubbles emit directly from the leaks, allowing you to dive down and fix them quite quickly.
I am confident from the amount of radiation he absorbed that Rad-X was effectively a requirement, at that stage. He still finished the quest with almost 50% radiation. It might be handy to keep some Rad Away on you, just in case.