around half of the pool is filled with poo
Humm, My pool does not get that way as you state in your question.
Johnny Seal goes by Johnny Seal.
The mechanical seal probably needs to be replaced, or it may be the seal plate gasket. if it is the seal it will eventually ruin the pump motor if not replaced. Go to you tube for a how to videoPOOLCENTER.com - Hayward Pool Pump Impeller-Seal Replace 2
If this blanket is vinyl as many are then use Aqua Seal or Seal All.
Its called Mastic. Call a local pool distributor in your area and ask for a professional recommendation. It is a tough job that even this seasoned pool professional leaves it to people who do pool construction. Luke Pool Service, Tucker, GA
Nigel Seal goes by Dart.
It sounds like you may need to replace the seal on the valve that controls the direction the water goes in.
Someone could use an inflatable seal for a variety of reasons. One might want to use it as a pool float in the pool. Another might choose to use it as a punching bag.
If it is the pump seal - replace the seal and other related gaskets. If it is the pump itself - replace the entire pump.
A leak happens
The o ring style seal goes on the lower groove, the valve keeper goes in the top groove.
As deep as seal is