These are the kayak industry standard, and we have used them for years in our rentals, lessons, maintenance, and personal kayaking. There are other more absorbent sponges, but they can sink which doesn’t work well for kayaking. These sponges strike a balance between absorbing lots of water while still remaining buoyant.
A sponge is a basic piece of kayak gear, you got to have one to keep your kayak clean and dry. For sea kayakers, a sponge can also double as a back-up bailing device if you loose your pump or it breaks. And remember to share -- kayak sponges are community property.
Note: These sponges get better the more they are used. A new sponge needs to be soaked and wrung out a lot to get the manufacturing residue out of it before it starts to sop up water like it eventually will.