Spa must be dry - the factory uses a hugely expensive epoxy. They call it Devcon, but I don't know which type of Devcon product it is. I have paid the price, cleaned the surfaces (sometimes I give the surfaces a scuff with fine sandpaper or a 3M scrub pad) and the thing stays in place forever.
If you want, you can try to purchase Devcon Epoxy locally and see how it goes.
If you want me to call the factory and try to track down the product type or number, let me know.
HTH