Even a strong swimmer gets tired in open water. Know when to float and rest on your back.

Being capable does not exempt you from running out. Open water takes more than a pool does, and strength mostly buys a longer stretch before the same tiredness arrives rather than immunity from it. Trouble tends to find the people who are genuinely good and have decided that rest is for everybody else.

Decide in advance what your version of floating on your back looks like: a day off, a shorter session, an evening with nothing in it. Use it before you are desperate instead of after. Being able to stop for a while is part of the skill, not a hole in it.