We made an island for turtles and frogs.
But it does not adjust to water depth.
We did somethings to help though.

We took builder's bricks, those concrete bricks
with the holes in them. Put them out in the sun and
washed them with the hose for a week (supposedly this
will help with whatever is in concrete that pond critters
don't like - someone help me with that something, what
is it?)

You might want to put down extra liner or carpet under
neath the bricks as they are rough.
We used concrete pavers to adust the height of the bricks, two bricks
side by side, topped by two more bricks, side by side, and then we
topped it with a big shallow planter. My husband says it looks like an
oil filter. No holes. You can find them for sale at pond stores or
online as water plant planters.

We filled the planter with soil and planted it with iris and lizards
tail, any marginal will probably work.
Around the edges we planted creeping jenny which grows over the sides
into the water and hides the planter.
Then we took branches, trimmed from the cherry trees that year, and
angled them from the edge of the island into the water. Our turtles and
frogs had no problem using the branches to climb onto the island.
You might have to fiddle with them for a bit, adjust and rearrange.
The water level changes from flooding the island to an inch below the
island, probably about the time I add water, once a week if needed
depending on the weather.

For a time we had a little wooden cabin on the island and a fake frog
fishing from the edge and a little boat tied up...

Now the island is overgrown, you can't tell it is not real. The fish
use the holes in the concrete as fishy condos when they are scared or
in winter when they are just lazing around.

kathy