Hi josie,
There is some information in post #89 here about the tradeoffs involved in different types of mattress protectors and the pluses and minuses of each.
If “waterproof” is your biggest concern then the types with a thin breathable membrane would make a good choice even though they may be warmer and less breathable than the other types.
Natura makes a wool protector which also has a membrane in it called the NaturaProtect Deluxe. I have one of these but if I had to choose again I would probably go with the St Dormeir which doesn’t have the membrane and is only “water resistant” for the sake of breathability.
My personal preference would be for a mattress protector rather than an encasement for the sake of ventilation unless there was a specific need for the encasement (such as high levels of allergy or an existing bedbug issue). Normal body fluids and smaller spills would be absorbed by the cotton and/or wool and wouldn’t go down the sides. If the amount of a spill was so large and in just the right place on the edge that it could run down the sides of the mattress then a protector that had a membrane or some other protection on the sides as well would be needed to protect the sides of the mattress. The Dormier for example has a stretchy fabric mesh that is used to fit it to the mattress that wouldn’t protect the sides of the mattress while the Natura has the membrane on the sides.
There are some very good mattress protectors available but which one is the “best” depends on the combination of benefits (including price) that are most important to you.
Phoenix