Using a Mifi device or tethering to your smart phone (the same concept) is secured through the encryption/authentication used between the device and your computer. This is the same for any 802.11 (wifi)device you use. If you use WPA as your authentication you'll be fine.
On our boat I have two methods of accessing the internet. The first is through the KVH satellite system. On this I have a wireless hot spot with WPA authentication. This is the expensive internet path, so I only use it when out of cell range.
The other method I use is to enable the personal hotspot on my Iphone. that provides a wireless hot spot as well. This is very cheap internet so its the one I prefer to use.
As far as security, there are two levels. The first evel is the encrypted link between your computer and the wifi hotspot. This is only a risk if someone is within the range of the hot spot (generally around 150'). Think about your boat. What are the chances of someone within 150' of your boat having the advanced computing capability to break WPA encryption? Not very likely.
The second level of encryption is the encryption between your computer and your bank. This is not going to be broken either. Yes it could be done, but it would have to be done by an IT professional that was directly connected to one of the routers between you and your bank, and who had the advanced computing power available to break the encryption. This just doesn't happen, thats why online banking, credit card machines, etc... are generally considered safe.
As far as the signal between your cell phone and the cellular data gateway at the phone company, I wouldn't worry about that. The equipment to monitor that is very specialized and you'd have to be within the reception range of both the cell tower you're on and your phone. Its just not worth it for someone to do that, especially with the encryption for banking as described above.
Now, here's another issue, and the issue isn't banking. Banking isn't the risk as described above. When connectiog to a public or free hotspot, that information can be monitored, and quite easily. All it would take is someone at the local router or switch using port mirroring and some monitoring software. They could see where you are browsing, and see any information you send in clear text (like your TF posts). So, just be aware that your free browsing might be monitored and act accordingly.