It is possible to prepare a test chart for underwater use. Film or Inkjet test charts can be enclosed in glass or waterproof lamination. Chrome on glass photomask targets can be fully submerged.
The main challenge of an underwater camera test is to present a clear image of a test chart, without any reflections on the target. This becomes very challenging when testing using a flat test target with a wide angle lens where on outer portions of the image, the angle between the camera and the test chart can exceed the angle of total internal reflection
Pawel Achtel has done significant work in testing underwater cameras, see his post: “Underwater optical performance of flat port, dome port“