There are many different options when it comes to natural springs to cool off during the summer months in FL.
Near Volusia County are: Ponce Deleon Springs, Blue Springs, Alexander Springs, Gemini.
Growing up we went to Ponce Deleon Springs and Blue springs pretty regularly. The manatees are a well-known staple at Blue Springs and can be found mid November thru March. It also has the largest spring on the St. Johns River, plenty of swimming, fishing, , snorkeling, canoeing & kayaking can be done here. Go early or call to ensure park entry due to high amount of visitors during peak manatee season.
At Ponce Deleon there is one large swimming pool that remains a constant 68' year round where you can snorkel, float in tubes (they do rent them there) or you can venture out in a canoe, kayak or paddle boat to explore the other water areas. Nature trails are also popular at this park.
Both of the sites below offer directions, park hours etc.