A Magellan Explorist will do what you want nicely. There are several models (100 to 600) as you go up you get color screens and more features, but I have used the 200 on my boat. I also have used the 500 model (color screen).
The 200 model will last all day on 2 AA batteries. The 500 can use 3 AAA batteries but will not last as long due to the color screen. You can hook up external power to these units if you want.
The Explorist model are weather resistant (IP7). The screen is a little small compared to newer plotters but still usable.
These models are obsolete but can be found on eBay pretty cheap.