A good choice for a defensive counter to both water and grass is Ferrothorn. Notorious for its abilities as a tank, Ferrothorn has a 4x resistance to grass and a 2x resistance to water. It can damage water types with its 120 bp STAB Power Whip and hits most things very hard thanks to Gyro Ball, which usually has ludicrous power due to Ferrothorn's low speed. Ferrothorn also resists Dragon type attacks, meaning it checks grass, water and dragon, the three types that resist water.
Another decent choice are any Dragon types. They all resist water and grass, and have powerful STAB moves like Outrage and Draco Meteor. Of the three I recommend the Haxorus line, as Hydreigon takes a really long time to evolve, and Druddigon, while decent, doesn't hit as hard. If you're willing to use legendaries, Zekrom and Kyurem are also great choices. It should be nlted that dragon types also hit opposing dragons supereffectively,
Finally Zebstrika is an interesting option. It gets both strong electric STAB moves and fire coverage via level up. Additionally, it is very easy to get, either as a blitzle early on or in the wild around the sixth gym. Zebstrika's abilities give it boosts when it switches into electric attacks, and lightning rod in particular redirects electric moves in double battles. This makes Zebstrika a good pair for Samurott, and Samurott deals with the ground types Zebstrika dislikes.
These are certainly not the only options. Galvantula is good as ThinMintE said, and I think Emolga, Leavanny and others would work too.