Yoga, my friends, yoga. Many of you may snicker but I dare you to walk into a yoga studio and take even a beginner class, it's harder than it looks. Not to mention that there are usually some attractive women wearing tight clothes in the classes (yes I am serious about yoga but I am also a single guy).
You're best best is some sort of hatha/flow yoga style like Ashtanga or Power Yoga. These types of yoga are a constant movement type of yoga (good cardio) with tons of core and pushups becuase of the flows (just like Mike's workout).
Not only will it keep you fit but it will help with balance, flexibility and focus, all applicable to boating.