Building on top of water

[Gazz]

With a sweet water lake I can place a lily on top of the water, then place a dirt block on top of the lily, allowing me to immediately start building on top of the water.

This would be quite useful with (often rather deep) salt water.
Is there a plant or other thing that I can place "swimming" on top of salt water?

[Shoom]

Thanks! That's such a neat trick I've never thought of, I've wanted to build a lighthouse out on this massive lake I got for a long time but avoided it because of the swimming it would take.