If you're installing the radio on a small boat, once you get above the bottom of the market, there shouldn't be much of a difference between the radios. Make sure you have DSC (all new radios do). Instead of worrying about the radio, you should focus on getting a good antenna and mounting it as high as you can. That will help you a lot more than a more expensive radio. Most radios probably come from the same factory in different cases. I usually go for Standard Horizon radios with Shakespeare antennas on the water, since in my unscientific survey, they've lasted longer for me. But try out a bunch of them and see what you like. If you have a small boat, make sure you get a waterproof fixed radio--don't want a splash to knock you off the air.
For a land installation on a 100+ ft tower, you simply cannot beat the IC-M604. I've worked with a coast station that replaced consumer grade radios with the M604, and the difference was definitely noticeable. It's receiver performance and intermod rejection are unparalleled. That being said, you probably won't notice that on a boat unless you're in an extremely RF-dense environment with a good antenna. There are better places to put the $350 extra you'd spend for the M604.
Since you're new to boating, I'd like to offer some general advice you using your new radio. Since you're posting on this forum, I'm going to go out on a limb and assume that you have the capability to monitor VHF from your house. Spend a lot of time listening before you start talking. Get to know what channel Towboat US, SeaTow, local marinas, vessel traffic (if applicable), bridges, commercial and recreational boaters use in your area. The link above is a good starting point, but actual use does vary from location to location. For testing your radio, I'd like to take DannB's advice a step further and say that it's inappropriate to hail any station for a radio check on Ch. 16. It can block out distress traffic, annoys the coast guard watch standers, and sounds unprofessional. For radio checks, see if a SeaTow automated radio check is available in your area. If not, hail a marina on their working channel, or try one of the non commercial channels. Channel 68 always has people monitoring in my area. Finally, get a MMSI number from SeaTow or Towboat US and interface your radio to a GPS following the instructions in the manual. That way, should you ever need to use your distress button, your position will be automatically sent to Coast Guard and they will be able to look up a description of your boat in a database...taking the search out of search and rescue.
Automated Radio Check Service | Sea Tow
Hope this helps.