Fundraising triathlons have enticed many runners to try to expand into areas like swimming , which they may not have learned to do very efficiently , to benefit particular charities , Mosca said . --Get a checkup to make sure you don 't have hidden heart problems . --Train adequately long before the event , including open-water swims -- not just in pools . --Acclimate yourself to the water temperature shortly before a race , and wear a wetsuit if it 's too cold . --Make sure the race has medical staff and defibrillators on site .