On those occasions when friends dropped off their dogs for me to take care while they went on trips, I separated the dogs in the house. The new visitor stayed in the dining room where I spent most of my time, and for the first day, my own dog stayed in the kitchen. I would introduce the new one, standing watch for any aggressive behaviors and step in fast. then let them sort out the sniffing and licking etc. For a day or two, I'd keep the dogs separated, then I would take them out together, with the visitor on a leash.
Usually, by the second or third day, no more growling occurred, except when the two of them started acting like puppies and went into wrestling mode.
This was with YB, the not-right dog, a GWP, and then with Mick, the late, great, ESS, both neutered males.
As long as your dog can be reassured that you are not abandoning him for a newer model, I've found that with you sharing yourself with both pups, no anxiety or hostility was exhibited. You are still the boss.
Depending on the protectiveness or insecuity of your Water Spaniel, should work out ok, if you do a decent job of introducing them.
YMMV.
Best wishes.