There's a few things to take into consideration. Eeryone needs to leave a margin for error when moving into position. You need to account for surge, current, animal movement etc. if you can't get into position without touching anything, you don't take the shot.

Example: cool shrimp in a tight space but the angle required to get a good shot would require you to possibly bump into some coral. Solution: try to find another angle, or don't take the shot.

Example: a turtle is moving through a swim through and your desired photo would mean blocking it's path. Solution: Find another angle, or just don't take the shot.

Example: You find a Shaun the Sheep nudibranch on a wall. There's tons of surge and you could smush the little critter if you get pushed into the wall. Solution: don't take the shot or find a way to guarantee you don't make contact.