I have no experience with a Slingbox, but I'll try to answer your question.
If your Slingbox can connect to the internet while it is at home then you *Should* be able to access it from your boat while on your MiFi. The big catch here is if Slingbox provides a cloud based service or if it requires you to log directly into it from an public IP Address. A cloud type service is more likely to work than something that requires a direct connection over IP.
I'd check with the Slingbox support forums for more info. Your MiFi is not going to need any additional configuration. Its either going to work or its not.