Tampons and swimming?

im going on holiday soon and im going to be on my period, um ive used tampons quite alot recently though im not too good at predicting when ive actually used them and i start leaking because i forget to check since im used to pads so when i go swimming on holiday when should i exactly use them and will i need to keep changing them if my flows havy, it wont fall out or leak or anything or the string wont show??? :/