Although some will contend the differences are insignificant, in fact there are numerous differences in the design and construction of marine radios vs. those intended for the amateur radio service
As Rick mentioned, frequency stability is one of them. Marine radios must meet a high frequency stability mark, which most ham radios do not. And, even if they can be made to meet that stability by using a high-stability time base option, this is only one of several requirements.
Another important one is spectural purity...the "cleanliness" of the emitted signal. Is it fully contained in the alloted bandwidth? How much splatter is there on adjacent frequencies? On harmonic frequencies? How well does the signal hold up with low voltage input? Some ham rigs will begin to FM badly (distort) and/or cut out when the supplied voltage drops much below 12.4 volts. Does the radio contain all the ITU marine frequencies in memory? Does it have an ALARM/Distress function on 2182? etc., etc.
Some radios are manufactured to be usable in SEVERAL SERVICES, e.g., the Marine Service
, the Land-Mobile Service, and in shore-based Fixed Station operations. Examples include the excellent little Yaesu System 600, which uses plug-in modules to "change" it's character to be compliant with the needs of the particular service. There are others, both new and used.
Admitedly, this business of what kind of radio can be used in what service is confusing. It MUST be, because it's been explained a g-zillion times over on this and other Boards. And, folks still don't get it.
Let me try one last time in VOA "Special English".
1. A radio used in the marine service
(i.e., on the marine SSB HF bands between 2 and 27 mHz), MUST BE FCC TYPE ACCEPTED FOR THAT USE.
No ifs, ands, or buts. You wanna use a radio on the marine bands and you wanna be legal
...you gotta have a type-accepted marine radio.
2. Any radio....repeat....any radio
, including those you buy, find, fix, build from scratch, steal from a storage
locker, or invent in your mind....can be used on the amateur bands for which you are licensed to operate.
You can modify an old military rig. Use an old marine rig. Modify an old aircraft rig. Anything. The difference here is that you are expected to know something about radio: you have to know enough to be responsible for your own emissions (transmissions), including their spectral purity, type, frequency, etc.
You with me so far?
OK. The confusion comes because ham radios are very nice little devices which can be quite sophisticated...much more so than, e.g., marine radios...and -- bye the bye -- can rather easily be made to operate on the marine frequencies. Wow. A cheaper, and in some ways more sophisticated radio which I can use for both ham and marine communications
? What a deal! Actually, that's what a lot of sailors do. Many of them do take the chance and operate illegally on the marine bands using a ham radio. Mostly, they get away with it. Mostly, they're pretty careful in how/where/how long they operate, though I know of one egregious case of a numb-nut who actually tunes his radio off-frequency to give his voice the "Darth Vader" sound. Clearly illegal. One day he's gonna pay.
Now, on to the more interesting question of can you beat the $2.5-$3K cost of an 802 with tuner, etc. Yes, you can. Easily. There are lots of very good used marine rigs on the market. At the moment, I have six of them on my shelves, one of which is used on a marine net every morning.
Sorry if some of the above doesn't track well....just getting over a 2-week bout with the flu...and I'm not tracking too well myself :-)