When making mashed pots I always boil until soft, drain and return to the hob to dry out completely. Then mash before adding butter, hot milk and a little parmesan.
If you try to mash the other ingredients into the potato, you will end up with something akin to plasticine. If you are making the mash as a topping, add two egg yolks.
No, they won't be watery, we do that all the time. A good thing to do with leftover mash is use it for making Shepherd's Pie, or, make fried mash to go with sausages and beans (one of our favorites!)
I shall be zapping it to buggery in the microwave! Th reasons why I asked is because I have put home fish cakes in the fridge and they were watery the next day.