triathlon in ocean water?

I only have one tri under my belt but I have done many ocean swims over the last several months and I know from experience that getting through the break is the hardest thing. I had my share of struggles with that when I first started training in the ocean. Nothing will expend your energy like sprinting into cold water and then holding your breath to dive under the waves. Go out slowly and remember the safest place is underneath the waves, even when your head wants to tell you otherwise. Once you're through the break, it's not that bad. The water has more movement to it than a pool or a lake but there's something kind of zen about it when you find your stroke and go. Oh, and don't forget to look back for the waves when you're coming back into the beach! The last thing you need is to be pummeled unexpectedly at the end of your swim!

As you said get out there and swim as much as possible in all kinds of conditions. As you know the ocean can be funny with conditions. Cohasett had 1-2 foot waves onshore at the start which was a mess. Get in there now that way its no big surprise come race day. Oh and get used to the taste of saltwater

The difference depends upon the type (size, location, wind patterns) of fresh water lake you're accustomed to swimming in. Swells, waves, chop, current... all of those things can occur in either setting depending on the conditions. Just get in plenty of open water time under various conditions and you'll be fine. My most challenging swim came from a fairly small fresh water lake on a windy day. I've done two salt-water races. Nothing happened there that couldn't happen in fresh water. Good luck with it....