You gotta understand they were ALL in the feels. Cillian Murphy may had it together the night before, but imagine that day after, then sitting there on the beach for good KNOWS when, and suddenly you are desperate and tried to take your chances. Plus trying to swim 30 MILES. Desperation make you do crazy things.
It's the only logical explanation. Everything in this film is linked one way or another, and after watching it 3 times, there was no better explanation. He was on that row boat that Tommy and Alex tried to get on and Gibson was ALREADY on. And that boat made it back. So Cillian Murphy was obviously back on the beach before heading back out. You saw other people desperately trying to row out there and you saw a man just go straight in.