Well, the majority advice would probably be ‘don’t’. But if you’re determined to challenge the Otter from Otterville to Rock’s Mills, or some portion thereof, you should consider taking:
Nothing that can’t get wet.
A watercraft that can take abuse from submerged and out-of-water rocks and logs – canoe or paddleboard perhaps preferable to a kayak, due to the amount of exits and entrances required.
Shoes that can take abuse/water. Due to the amount of submerged walking amidst rocks on unstable streambed, over trees and potentially, portages, an old, but still solid pair of running shoes are probably the best. They will get wet. Your feet will not be wildly comfortable for the duration. It’s just the way it is.
A good baling utensil – even if your boat isn’t leaking, the amount of water coming in from repeated exits/entrances requires attention.
Bug juice – depending on your tolerance, either repellent or a bug shirt may come in handy.
Sunscreen, a hat and layered clothing combining protection with the realization you will most likely be soggy from the waist down. Again, that’s just the way it is.
A strong back – you will be dragging or portaging your craft for a considerable chunk of the journey.
Lots of water or other drinks.
Waterproof camera to record the fun.
Cellphone – it would be eminently possible to sprain an ankle, for example, or worse. Having a Plan B/back-up/emergency extraction plan in place is a very solid idea.
Positive attitude – it will be needed and tested.
Victory Doritos – your choice of flavour.