Also, Sharon didn't see the sign, but Rob did. It was a fucking murder-suicide!
Wow, that's the absolute darkest interpretation possible. I assumed he wanted to get out to Sharon and get her to come back in without panicking her. I did also think that Sharon might have been less likely to go out to him, risking the children, if it was the other way around and that possibly is because Rob has the more nihilistic streak of the two of them. But it is odd that he recognised how dangerous the beach was and still took time to kiss her and then mess about laughing, instead of telling her to get back in asap and be prepared to swim to the side rather than against the current if caught.
I think it is ambiguous as to whether they get out or not. At the end, they were still swimming in the direction of the shore but not getting anywhere, which could have implied that they were caught in a rip current. The cinematography choices definitely pointed to them being lost at sea as the last shot from land was of their clothes as they faded to a blur of water. They both seemed to be moderately strong swimmers, so they could survive a rip current. My dad was caught in one once and he made himself not panic and swim to the side, out of the current and he was fine. Sort of terrified but fine. But Rob not telling Sharon of the danger makes her less likely to have the presence of mind to swim sideways.
If they did both drown, both kids wouldn't necessarily die. Frankie is old enough to get himself out of the car and eventually have someone help him. Muireann is probably doomed unfortunately as Frankie wouldn't understand the danger and wouldn't try to move her unless she cried and she's more likely to be in the deeper sleep from the heat so wouldn't.
If Sharon and Rob get out of the sea, I don't think she should move to the US with him. There is no way I'd willingly move my children abroad with a man who had threatened to take them without me. If/when they inevitably divorce in the US her options would be severely limited. And even worse, Rob is an alcoholic with a history of relapse and rewriting history (he relapsed before Sharon and the student, not because of it as he keeps claiming). I've known too many parents to be trapped in the country their children reside in, to ever be willing to move my dc anywhere that I wasn't 100% sure I could happily live forever. I might forgive Rob for his horrible outburst after the funeral, and I might be willing to give him a chance to stay sober. But I'd only ever do that where I had as many options as possible to keep as many freedoms as I could to protect me and my dc in the event of a break up.