Well, there isn't much you can do really... You would have to make a custom model, depending on what you want your item to do.
There are two things you can have, the item "sitting" on top of the water, or the item looking like its "floating" on the water, with an actual floating animation. Now to do the second one, you would need to do a little work.
Take one of the models in wc3 (say the floating lily pads with an animation) and remove all the geosets from it, and then get the geosets from the item and import them into the lily pad model and attach them to the lily pads bones. Then move the geosets around (if there are more than one of them) until they fit together again, and there you go...
This isn't easy, so I recommend you to do the first of these two, even if it won't look all to good.