Please note that these hatches have a relatively light surround and should only be fitted into a perfectly flat surface which is free from flexing otherwise they may not seal properly. While these hatches are water-tight, they can not be used under or near the waterline and similarly they cannot be used to hold back fluids. While they can be installed in the top of a water or waste tank (but above the fluid level), they should not be installed in a fuel tank as the o-ring may become degraded by fuel and no longer function.