The Icom 706MKIIG is the most popular ham transceiver....ever! It is an excellent little radio
for use on a boat, and can do email
I generally like Yaesu equipment, too, but the 100D has been problematic in several instances, so I'd avoid that one.
There are lots of other choices, too, in small to medium size ham transceivers.
You will need an automatic tuner. I'd avoid the SG-237, but the SG-230 is the gold standard and has been for the past 25 years. It will cost $500 new, and I'd be very careful about buying
a used one.
Your $600 budget is not realistic for both a radio and tuner, especially with these choices (IC-706 and SG-230 tuner). A modem
for email will cost an additional $500-800 used.
It is possible to find a good used marine SSB
transceiver and automatic tuner for not much more than your $600 budget. I have several. PM me for details if you like.
There are several alternatives to a backstay antenna. On some boats it's possible to fashion an "alternate backstay"....a robust and low cost alternative... which doesn't require the pricey insulators.
Some antenna choices for maritime operation are shown in the following chart: MarineAntennaChoices2