You don’t need to know how to swim. If you are going on the water you will be wearing a Personal Flotation Device (PFD) which will keep you afloat if you fall in. The Boyne is waist-high in most places so you can just stand up when you go in.
A reasonable level of fitness is needed for outdoor activities but you don’t have to be Brian O’ Driscoll. Back rests are available on kayaks if you suffer from lower back pain. Swallow your pride on this and request one, trust us.