we used it on our first
DCL cruise - to swim at CC.
You have to apply it an hour before you go into the water for it to be effective. We didn't like the 'feel' of the lotion on our skin. It has a very 'thick' feel to it. And it's difficult to wash off. However, we didn't get sea lice and others on our cruise did (a very irritating rash). So it seems to work.
We only swam for a very short time at CC on that first cruise, just to snorkel a bit. We didn't like the water there at all (we live 10 minutes from a beautiful beach with much nicer water, so what's true for us might not be true for you).
We were in maybe a half hour to an hour at most first thing in the morning. Then we spent a good 15 minutes in the CC showers trying to get the lotion off of us. It wasn't easy.
On our second cruise we didn't go into the water (other than for the banana boat ride), so we didn't bother with the lotion.
I'm coming back to add one more thing regarding jellyfish.
Jellyfish are a much more serious problem than sea lice. And this product is well known to protect against jelly fish (although not the portuguese man-o-war). If there are actually jelly fish when you're in Mexico, and you plan on swimming, then you might seriously consider using this product. Keep in mind that you MUST put it on 1 hour before entering the water for it to be effective.
Personally, when it's jellyfish season where i live, i stay out of the water (i hate those ugly buggers). But i know people who do use this lotion where i live when jellyfish are in season.