$5000 a month? That's pretty embarrassing, isn't it? That indicates they aren't doing satellite-to-satellite and are using some kind of specialized hardware to simply send the signal to coastal satellites from farther away.
Inmarsat is the only viable alternative for smaller boats that offers unlimited data plans, has higher latency due to being geostationary, much lower bandwidth, and charges about $8000 for a gigabyte…
I‘m not sure what Ku or Ka band GEO providers charge, but I doubt you can find anything competitive there either, and these require very large antennas.
350mbps is _insane_ for this
Your actual cost is irrelevant.