Take a look at the clarion. I think it was one of the boat places (not Boat US) but maybe West or another that had them BELOW dealer cost!!!! Otherwise any of the clarion single din units will fit the bill. Another option is to get a cover and then you can use about any head unit you want to. Clarion also makes a fully waterproof head unit that you can actually dump water on and it is fine but that will get ya back up to the 400 buck range. All the clarion remotes have the remote jack so you can put in a guage size remote in the dash, or even on the transom of the boat. If you go with the cover then you can go with say a sony and still be able to do the remote.
For the amp get a QUALITY 5 channel and you will be fine. I use the Rockford the most to fit this bill. It has good power, does not use TONS of battery to run and is reasonabley prices for what you get. I am sure there are other options for you in your area. Stay away from amps made by Sony, Clarion, Kenwood, and the alike.
Hope all that helps