Playing Fallout 4 on Xbox One, some of the quests require you to swim.
For example, in Thicket Excavations, you need to swim underwater to repair pipes. Unfortunately, the pipes are quite a bit underwater, and the water is irradiated. Sully Mathis suggests you look for the bubbles, at the surface, to identify where the leaks are. However, I still die before I fix the pipes, in part due to difficulty with the swimming controls.
What are the swimming controls?
To swim, simply look in the direction you wish to move, and move forward.
It might also be benefitial to pick up issue 7 of the Wasteland Survival Guide, found in Old Gullet Sinkhole, as this will reward you with a perk that increases swimming speed by 25%
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 amount of radiation he absorbed. You could also go for more permanent protection, such as lead armor, or a reinforced Vault 111 jumpsuit. It might be handy to carry a Rad Away on you, just in case, and have it mapped to the d-pad.
You will notice an elevator of sorts, further down past Sully. If you press the button, the elevator will rise up, although it takes a little. Inside this elevator is a lock box with a Novice lock, holding a Rad X.
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.
The valves are yellow-coloured, and stand out from the pipes. If you follow the pipe directly down, they are impossible to miss. You simply have to use them, there is no hold down or turn animation to wait through. The second I hit the valve, I am moving on to the next, or surfacing for air.
The swimming controls are not different from the regular moving controls.
On XBox One, you'll use the two sticks to move, and that's it. Look in the direction you want to move, and go forward, backward, left, or right. If you want to swim upward, look up and press forward on the movement stick. If you want to swim down, look down and press forward on the movement stick.
There's no separate control for diving or surfacing.
Note that you sink in Power Armor, and can't swim. You can walk on the bottom, and don't run out of air, though.
I've done the quest you're referring to without too much trouble, and I've had to swim for other reasons, including retrieving equipment off a raider that fell into the river when he died. Find your objective, aim directly at it, and press forward. The bubbles really do indicate where the pipes that need fixed are.
When you're near the surface, you can jump to help get yourself back on dry land.
To answer the question directly stated, you swim with the same controls as walking, simply orienting your view in the direction you want to move.
To answer the question of why you're dying before you can fix the pipes:
As a quick and easy workaround, you could wear a Hazmat suit. This will pretty nearly prevent all radiation damage.
I know of three Hazmat suits:
Please note that this will not stop damage from lack of air - despite the apparent air-tanks on the back of the suit.
Alternatively, you can find and consume some Rad-X, which will increase your radiation resistance by +25. This will prevent some radiation damage - possibly enough to allow you to complete your task, and should last several minutes. This will also be more effective if you have perks.
Alternatively, you could get the first level of the perk Aquaboy (/Aquagirl), which provides the following benefits:
Water is your ally. You no longer take radiation damage from swimming, and can breathe underwater
This will require 5 Endurance, and a spare perk point.
Don't use armour suit, make sure to use rad remove.
