If the high is 70, it will not be warm enough to swim, unless your hotel has a heated pool. It's difficult to tell when the 'best' time is--we were there last February and it was perfect! Beautiful weather, moderate crowds, very few ride closures. That said, I've been to DL in Feb when it rained heavily and the park was PACKED. You can count on summer being HOT, but any other time can be very comfortable. Of course, there are no guarantees. Summer, weekends and holidays will have large crowds. One of my favorite times to go is 1-2 weeks after Thanksgiving. The crowds are light, the Christmas decorations are up, usually all the rides are working and hotel/transportation costs are as low as they ever are.