Can Aquatabs be used to clean swimming pool water?
Aquatabs can certainly be used as a disinfectant in a small swimming pool like you've described. The active ingredient has many uses, including drinking water treatment as you've pointed out, but it is also used as a disinfectant in swimming pools.  

According to this Wikipedia article regarding the active ingredient of Aquatabs, sodium dichloroisocyanurate:  


  As a disinfectant, it is used to sterilize drinking water, swimming pools, tableware and air, fight against infectious diseases as routine disinfection.  


A study was done[1] comparing the effectiveness of sodium hypochlorite (common household bleach, also used for pool chlorination) with sodium dichloroisocyanurate. From the abstract:  


  A comparison of commercial sodium hypochlorite (NaOCl) and sodium dichloroisocyanurate (NaDCC) products was made. Solutions of NaOCl and NaDCC containing the same levels of available chlorine (av.Cl) exhibited very similar bactericidal activities, despite significant differences in pH.  


The ideal level of active chlorine for a swimming pool at neutral pH is about 0.5 to 1.5 mg of active chlorine per liter of pool water. So, if you aim for the middle of this range, you would want to add one Aquatab tablet for every 300 liters of water in your pool. Also, note that chlorine is less than half as effective at a pH of 8 as it is at a pH of 7, so I would pay some attention to the pH anyway.


D. Coates, J. Hosp. Infect. 1985, 6 (1),31-40. A comparison of sodium hypochlorite and sodium dichloroisocyanurate products. DOI: 10.1016/S0195-6701(85)80015-3