Since you specifically say you just want to have fun, I would say, "Yes, you can do it.*"
* Note - Expect to be really really slow on the swim!
Tips for the uber- beginner/first time swimmer:
* Go slow at the beginning. Really slow. Don't even try to keep up with anyone else.
* Relax, relax, relax. Don't try to go as fast as you can, and try not to panic.
* If you go too fast, and begin to panic, roll over onto your back and catch your breath.
* Use any stroke you need to. You can breast stroke, backstroke, or even dog paddle.
* Don't forget to breath.
* Don't worry about your speed/time in the water. Just enjoy the swim, then hit the ground running.
400m will look really long in the open water if you've never done it before. Don't freak out when you see the distance. If you are an average swimmer, it will probably take you 8-10 minutes to swim the 400m on a good day. If it takes you 12-14 mins, so what? You'll make up for it on the bike and run.
Don't worry about embarrassing yourself. You won't be the slowest swimmer, and there will be lifeguards. But you do need to keep in mind that if you are uncertain about being able to stay afloat in the water, and you think you are going to need help after 2 minutes, then consider skipping the race. It would be foolish to get in the water without being able to swim, because you put not only yourself at risk, but you put others at risk when the lifeguard has to come rescue you.
Just my opinion. If you decide to do the race, Be Safe and Good Luck!