You should be aware that 2m/440 packet bbs & email links aren't as fast as most landline email, including dialup. I have no idea how fast or slow the mail is for mariners, as I haven't tried that due to extreme expense.
I found that one reason for this observation is that the vast majority of packet radio is stuck at 1200 baud, with maybe 1% using 9600 baud data rates. Again, it's a cost issue. The folks selling such items simply priced themselves out of reach of most folks, thereby contributing to the demise of an other wise excellent mode of communication. Add to this the wide variety of modulation schemes, the need to modify your radio to accept 9600 baud and up, and you can see the only downside of this mode is expense and availability. While you can buy a 1200 baud TNC and compatible radio for $100, you'll have a hard time finding anything at all that's even compatible with higher data rates, as well as finding the TNC for it.
A notable exception to this, of course, is the old Tigertronics modulators, which are plugged into your computer serial or parallel ports, and then hooked into your radios. These can run quite well and cheaply. Any PC will do the job....even an old DOS PC like the handheld HP-95/100/200lx series, or the Omnibook 300 (386sx). I've even used old handheld PC's like the Palm and newer units as terminals.
I'm not trying to discourage you here, but enlighten you as to further options. In an emergency, packet radio can be a life-saver, especially if coupled with APRS and a fly-over by one of the pacsats or ISS. 5-25 watts will make your connection to these birds, and then the entire continent will see your signal in the downlink. Way too cool, in my opinion!
By the way, while a Kantronics TNC will do great work for you in this mode, don't overlook the older micro-TNC's like Heath Hk-21, Tasco, and Pico/Handi-Packeet TNC's, all of which are still top-notch.
Enjoy!