Bullseye boat blast

Are you talking about the Gnome target shooting down at the docks?

There's nothing special that you have to do to start that except to talk to the Plant/Zombie character down in front of the storefront to the right of the docks to start the challenge. Regardless of what character you start off as the challenge will place you as a Cactus or a Pirate depending on what team you were on when you started and you simply have to shoot the Gnome targets as they pop up.

You will get cactus or capt. Deadbeard.

Use spyglass shot or spike shot(zoom in). Destroy the gnomes!

You have to go to the scuba soldier or corn.

First finish the kernel corn's and the scuba soldier's quests. Finish each 3 quests and talk to them again to start the boat blast of destroying as many gnomes as possible. Tactics for getting high scores: go for time freezing gnomes if you can, go for double gnomes if you can hit them easily, go for stationary gnomes after double gnomes so you don't miss them as easily as moving ones.