You can as well use a boat to go to open sea, and swim a little bit. I don't recomend taking a swim in the river, lol. Not this one at least. But you can go swimming at the beach or, as I said, use a boat to go farther away. Though you might have problems.....
Don't tell my aunt you saw this photo, lol.
You can also take a ride on a boat along the river. You can take it at Deltebre, for example, where it will get you to the same end as someone goes on talking giving you a tourist explantion about the river, and some history related to it. On the other hand, you can take your own boat or rent one. There was a time when my father bought a little boat. He named it 'Yaiza' (after a little town somewhere in Spain, don't remember now). We usually rode on the river on that boat, and while I was fishing he was practicing kayaking. (Here we are almost at the end of the river in one of those plaes you need a boat to get there, and that's an old friend)
In the Natural Park there are many things to do, nevertheless the are some I don't like people doing cause they endanger the environment. So I won't say them. ;-) But I'll do tell what you can do respecting the environment.
First of all, and as in any Park, you can sitesee. There are many places to see although transportation is pretty rough in the Park. For example, you can go to the north side (I call it, though it's called the left side, lol). You can go see the flamingos, ilusions (though you can see these all in the Park) or the Headlight. But hey!!! don't miss the fabulous sites, lol.
But if you get boatsick, then don't worry, you can rent a bike and keep on with the siteseeing story, lol. Bikes are usually rented at the camping sites, but don't you think they are those last model mountains bikes. No way, they are sort of unconfortable (rather say very!!!). Watch out with your bud!!!
If you like fishing go ahead, but remember don't keep the little ones, they still have to grow!!!! There are several ways to fish! You can try it from the shore, go on a boat (two modalities), or even use a kayak. Almost at the end of the river you can rent a boat. There is another interesting modality, that is, going to those mussel shelters you can find at the Delta, and fish from there using crab as a bait. The problem is that you never know if you are in the right day to go fishing, lol.