This answer is VERY broad, but, from what I've seen/know...
If you're talking about something that actually puts you out on the water, you're pretty much just going to be limited to harbor jobs (harbor tugs, ferries, etc). The availability of those positions varies greatly by location. For example, it's not unusual to see younger crews on some of the assist boats in NY Harbor or companies like Starlight in SF Bay. Up in Puget Sound, however, the guys on the assist boats with Foss and Crowley seem to all have 20+ years in the company. Those jobs are very hard to get and they do work a 2 week rotation.
Frankly, if I were to do what you're thinking, I would want a rotation longer than two weeks. You'd probably get burned out between work and the ANG.
Deep sea (ship) jobs are generally crewed for each voyage, with rotations happening in the U.S. A typical stint may call for anywhere from two to three trips (90-120 days). Some ships may have longer or shorter voyages. Coastwise vessels (Jones Act Trade, ATBs, line-haul tugs) usually work shorter rotations and there is much more flexibility on where/when you can rotate.
A lot of this also has to do with if you're going to be sailing out of a union hall, or can land a permanent position with a company. Also, keep in mind that your first job after graduation is most likely not going to be with the same company or even same sector that you'll spend most of your career in.