i have got sea lice in the past, which is actually the small barbs from blue bottles, gives you like a red rash, nothing some white vinegar wont fix, as for fresh water there could be anything living in any body of water. i dive the larger cleaner lakes, and try to avoid anything that looks questionable. and i always put the ear drops in my ears after a dive. it think its alcohol, all dive shops around lakes will have it in stock, it also dries your ear canal.