Whether your "sea lice bites" are actually baby jellyfish or sargassum seaweed dermatitis, after five trips, we've never felt like we needed to pack special medication just for that. A general topical analgesic and some hydrocortisone, which you are probably packing anyway, will do just fine.
Vinegar helps neutralize jellyfish venom (no, don't pee on the sting), so if you feel like you must pack a jellyfish-specific remedy, you could just fill a little travel bottle with vinegar.
Perhaps
@PrincessShmoo can speak with authority on this, but I have to imagine the infirmary probably has something on hand, too.