You don't even need the modem if you are not interested in high fidelity (read attachments and big files) email. if all you need is text then there are lots of ham only solutions that use a computer modem sound card and ham transceiver.
If you want to talk on the marine bands then you need a marine radio. The have to be certified and prayed over by the FCC and others so they cost more. Also, most ham radios can receive on the marine bands but have the transmitter locked out. Kinda useless if you need to use the marine frequencies. The only thing I can think of needing a marine freq for is buying parts while at sea. Cant do commercial transactions (mostly) on the ham bands. In an emergency you can use the ham bands to summon help. There are several dedicated nets for this purpose. This is not the case for the marine bands to my knowledge so the ham bands would seem to be the better safety item.
So it really depends on what you want to use the radio for.
If you want to talk on the marine bands then you need a marine radio. The have to be certified and prayed over by the FCC and others so they cost more. Also, most ham radios can receive on the marine bands but have the transmitter locked out. Kinda useless if you need to use the marine frequencies. The only thing I can think of needing a marine freq for is buying parts while at sea. Cant do commercial transactions (mostly) on the ham bands. In an emergency you can use the ham bands to summon help. There are several dedicated nets for this purpose. This is not the case for the marine bands to my knowledge so the ham bands would seem to be the better safety item.
So it really depends on what you want to use the radio for.