You could try applying a preventive lotion or cream* to exposed skin before entering water.|
*While not scientifically proven to be effective, there are several products people have reported they use to try to avoid swimmer's itch; with varying results. They include: Swimmer's Itch Guard (cream), Bullfrog (sunscreen), and Sea Safe sunblock. A barrier layer of Vaseline has been used effectively by some, even “Crisco Shortening”.
However, these reported results are anecdotal and ILA is not endorsing any of them. You should check each for ingredients before trying and talk to your doctor or pharmacist before using if you have any questions about their use. Report back on your results if you wish to share.