I won't be surprised if we discover/find other creatures we haven't seen before in the future. Since we have over fished the shallower parts of the ocean fishermen have started using more deep sea nets to try to get the same quality of fish that they've had in the past. It will take much longer for those deep sea fishery grounds to recover since things just grow much slower in cold water.
If you've seen The Blue Planet series, you'll realise how much of the ocean is left uncharted. They discovered new species while doing the documentary. Definitely great inspiration if you're doing an underwater campaign.