...i basically want to create an underwater forest, and was wondering if there is any way to either simulate a tree, or if there are actually any bonsai species whose roots and or foliage can remain submerged.

thanks

there are no underwater "trees", but there are water plants like arrowhead, hydrilla which you can attach a stone or pebble to, to make them stay submerged all the time

No bonsai trees can do that. You could get a piece of deadwood and creatively attach smaller aquatic plants on it to kind of get that effect though.