the latext gloves go up to your wrist. If you put your arm in the tank, the water will get in between the gloves and your hands and your hands will get wet. My main concerns was not so much keeping my hands dry but to keep me from touching the rocks and corals with my bare hand. If I have something like sea urchin or a lion fish where the spines can be poisonous or extremely painful if touched, I would definitely wear my heavy duty arm length gloves.
If you have zoanthids in your tank, it might be a good idea to handle it with gloves as I heard that the chemical it releases can pose a hazard to your health if inhaled or entered your bloodstream from cuts on your fingers. Other than that, I don't know of anything else in your tank that would be considered as risky.
James