That's a very good idea! As you may know, sponges feed by filtering seawater through many tiny holes across their whole bodies. Because of this,they may indeed be very good indicators for how much of various pollutants are present in sea water. Their cells are generally very "plastic," meaning they can perform various roles, and often even change from one cell type to another cell type (like human stem cells). Because of that, it might even be possible to cut pieces of them off to test for the toxins instead of collecting and killing the whole sponge. The piece that was cut off might easily regenerate.

My guess is that they would probably work well for some pollutants and not for others, because different chemicals will react differently with sponge cells and proteins. Scientists often have to "optimize" their experiments,which means they have to try different possible. In this case, you'd probably have to optimize for what sponge species worked best and was easiest to care for, and what chemical pollutants that sponge was best able to absorb from the sea water. After that, you might have a great indicator for ocean pollutants!

Keep coming up with such great ideas!