Check out Byram's link to get sticker shock, especially for larger hulls. . You still need room for the unit as well as extra batteries and associated charging wiring. The units are pretty expensive but more importantly, the install can be costly as is the required yearly maintenance that starts at over a grand and up depending on the model.
I don't know if there is sufficient room in a Marlin.
I saw a 305 Express for sale that had one installed in the aft birth but for some reason they did not show any pics of the seakeeper. I would assume it basically eliminated the entire aft birth Section.