Try giving your Samurott fresh squid on a regular basis. It’s a great way to keep them hydrated, a good source of several vitamins, and a low-calorie snack.
If they refuse squid, try unflavored gelatin. It doesn’t have the nutritional value of squid, but it will provide much needed hydration (older Samurott are prone to kidney problems, and keeping them hydrated in youth can prevent some of them).
But do squid exist in the Pokemon verse…? Or are you instead suggesting we feed them ground Tentacool?