There is a shuttle from Sea World to Aquatica,it is possible to walk between them but I wouldn't call it easy.Never have walked between them but I did take the shuttle once and before taking it looked into how walkable it was between them,you basically have to reach the sidewalk outside the Sea World parking lot on Sea Harbor dr, then cross the very busy International Dr which will take you by the car entrance of Aquatica but it doesn't show to have a sidewalk in this way so you have to walk south on I Drive to where the cars exit Aquatica and go in through there,doable but getting the shuttle outside Sea World seemed easier,especially in the heat.