Some pool heaters have an internal bypass valve that improves efficiency by limiting the amount of water flowing through the heat exchanger. These can stick so that water flow mostly bypasses the heat exchanger...the heater gets hot but very little heat is transferred to the pool.
Other answerers have suggested the following:
Try a pool shop
You use a water heater, which may be electric or gas, to circulate the pool water through and maintain temperature.
no you need a stronger heater specifically designed for a swimming pool
That would depend on which heater model.
You can buy a swimming pool heater in the Burlington, Vermont are at Leslie's Pool Supplies. They have a great selection.
Acidic
Unless you have an ozonator, which would inject ozone (O3) to sanitize, there are no gases added to the water of a swimming pool.
81F
I would not suggest a water trampoline for your home swimming pool. You can not control the trajectory when you jump and injuries could be caused if you strike the pool skirt.
To keep your swimming pool clean you will need a filters and a water pump. A vacuum would as be a good idea.
Unless you leave the heater on, Yes
Swimming pool water can be what ever temperature you would like it to be just not too cold or too hot just need to have a little relaxation. :)