When you're spending your summer vacations in the town of Šibenik, or in the nearby coastal towns, you can enjoy in the unique experience, swiming in the salt-water of Adriatic and sweat-water of NP Krka. Usually, the sweat-water is colder then the salt-water, approximatelly for 2-3 C degrees. Therefore, before entering in the water make sure it is not too cold.
In Primosten, about 30 minutes by bus from Sibenik, there are two fairly large beaches - they both are free beaches - and are surrounded by shady pine trees. The water is as blue as it can get - and the beaches are very popular with friendly croatian families.
Equipment: the beaches are free beaches, hence there are no facilities there - so you have to bring your mat or deck-chair. A sun-umbrella since there is enough shade from the pine-trees that surround the beaches. They are rocky beaches, so u may want to bring some "swimming" shoes
There are several hiking possibilities at Krka National Park - the one I'm suggesting is a very short and scenic one... it should not take more than 90 minutes to complete - it's the one that leads you through the forest down to the foot of the big waterfall, where you can go swimming with the locals. From the watermill museum at Skradinski Buk, where you can catch your first view of the waterfall from high up, you see that there's a well-marked forest trail: it's the one you should take. it twists and turns through small lakes and waterfalls until it reaches the bottom, then over a wooden platform it takes you over the big waterfall and the shore, where everyone seems to go to swim. It may look dangerous, but it's not - or else hundreds of other people will not be doing: they're locals, so they know how safe it is.
Equipment: the hike is easy - just wear comfortable shoes, preferably (but not necessarily) hiking boots
Skradin is a sleepy village just outside Krka National Park - and has a nice concrete beach with wonderfully warm water - after all it's in a lake you'll be swimming, not in the sea. But i found it very pleasant - and the people were really nice there. To get there you can go by bus from Sibenik, but the whole point is to go there ater visiting the krka NP. From there, there are hourly boats leaving from Skradinski Buk that take you to the villageof Skradin 20 minutes away. As an added bonus the cruise is nice and scenic, with great nature and passing under a bridge where local kids gather and, as soon as a boat has gone past, they'll delight you with the sight of their acrobatic jupms from the bridge into the lake.
Equipment: swimming suit and towel, as it's a free (concrete) beach