As a material, acrylic plastic is actually worse than water (refractive index about 1.49 vs 1.33). But thickness is key. With only 10mm of the stuff to look through, there would be no significant spherical aberration at the magnification and aperture we're talking about here.
By the way, there's much more discussion of this topic (shooting through water) in the thread
HERE. It wanders around a bit, but there's solid technical content and image comparisons on the later pages.
One aspect I forgot to mention is that it's important to keep the central rays of all the entrance cones as perpendicular to the glass as possible. That way you only run into spherical aberrations that diminish as aperture to the fourth. Cones whose central rays are not perpendicular to the glass suffer from other aberrations that do not diminish as quickly. So it's better to shoot with a long lens or any other setup that is closer to telecentric. Shooting with a short lens that has a wider angle of view will pick up worse aberrations away from center.
--Rik