Sea squirts or tunicates are not underwater plants but marine invertebrate animals. They are also called ascidians and you can find them in salt water all over the world.
The interesting thing about tunicates lies in their anatomy: In their larval state, a sea squirt looks a lot like a tadpole – swimming around in the ocean. When sufficiently developed, the larva finds a suitable rock and cements itself in place. They then pump water through their bodies (in with the bigger hole on top, out with the smaller hole on the side) and filter out plankton.
All have a harder outer shell (the actual tunicate, left picture) and a filterlike inner body (right):