Even 3-10 MB could be painful.
Are you not going to have Wifi when you're on land?
Not sure how you'd get consistent mobile data on a ship.
I think cheapest Mifis are still around $100, with poor battery life. Maybe look for a second hand iPad with 3G or 4G, which will provide much better battery life and more band support.
For instance, I used to use a Huawei Mifi from eBay and it got 3G okay in Europe but not in the US. Battery life was 2 hours at the most.
The iPad mini with 4G gets LTE in Europe and in the US and battery life is over 10 hours.
But if cost is an issue, I'd look to get Wifi access, though Wifi in many places can be pretty slow.