engineer,
You really should talk to your DS/DD about transportation, as they will best know the way USNA will get them to/from Groton.
If MIDN are bussed to/from Groton, any air travel you purchase is not reimbursed (USNA did not purchase air travel for MIDN, i.e. gov't bus was available to/from Annapolis). I wouldn't be surprised if this was the method as MIDN are frequently bussed between Annapolis and Newport -- but like I said, talk to your MIDN.
Southwest closest airports are Windsor-Locks, CT (Bradley - Hartford/Springfield) and Providence, RI, which is a bit of distance from Groton. Your MIDN will have to work closely with their command to see if a duty van or ground transportation service (shuttle/taxi) is available. This is a great thing for him/her to look into. I would also default to finding a suitable ground transportation service and NOT the duty van.
Also...do you know if your DS/DD is on leave immediately following? If so, it is common for MIDN to leave from their cruise site.