Idk if the land mass is enough for a full grown tiger. They get near 8" long and are way more terrestrial than aquatic....
Don't want to be a party pooper here, but I agree. Tiger salamanders are a type of mole salamander. Probably the reason they spent most of the time in the water for you is because the substrate you provided them was rocks. They had no where to dig or hide.
I used to have a pair and kept them in a 7 inch deep mix of coco fiber. They spent about 95% of the time underground for months on end. Not the most interesting pets...
At any rate I would suggest you try a "forest floor" type of terrarium with a thick substrate layer and see how they react to that.
Wiki quote "Adults are rarely seen in the open and often live in burrows that are usually 2 feet from the surface. Tiger salamanders are almost entirely terrestrial as adults, and usually only return to the water to breed."