A good way to workout and keep your core muscles in tune with the motion they are active when kayaking is to get a pilates/yoga ball, a paddle/weight bar/broom stick/etc, (I use a 9oz straight bar in the gym). Sit on the ball with good kayaking posture (sitting straight up-not slouched, good hand position on the bar or whatever you're using to simulate your paddle, feet/legs in the best position as possible to simulate the way they are in a kayak--> not gonna be perfect being that you're sitting way up on a ball, etc.) From here, concentrate on using good form and technique to just activate your core; rotate your core in the manner you paddle. I do 10-20 on each side. JUST CORE here first while holding your "paddle" correctly. At the end of those reps, I move straight into paddle strokes, again 10-20 on each side. Slow, and concentrating on good form. I do about 3 sets.
More CORE:
Pull ups: The only change here is to Bring your knees up so that they are parallel to the floor (Bring your knees up to your waistline-ish) From here, do pull ups as you normally would while keeping your knees up. This keeps your Core activated.
Dips: Overall great exercise for a lot of your kayaking muscles; shoulders, tri's, chest, etc. * Like above on the pull ups, keep your knees up, parallel to the floor while doing your dips. This again will keep your core activated. If you need help/spot, put a pilates/yoga ball under your feet to push out more reps.
Work all of your muscle groups, don't neglect any.
Cardio, Cardio, Cardio. Helps a TON.
My "Off" Season activities:
- Gym (Working everything) cardio in the gym, I use the recumbent exercise bike (less stress on knees and back--> my knees and back are a bit jacked up, jumped out of planes for a living and humped quite a bit of weight/gear) and as an alternative to the exercise bike and running the track, I do a bit of Krav Maga for about 30mins to an hour.
- Play Paintball
- Shooting real firearms at ranges
- Hiking (Incline if you're ever in the Springs/manitou area)
- Ruck Marching with about a 65 lbs ruck sack. (at least ~5 miles) ( In a way, I guess this could simulate humping your kayak to the put-in or to the take-out)
Fitness for your river mind:
- If you can, and if rivers are still somewhat flowing, get out and hike along a section of river. Take time to read the river and rapids that you may never have looked at from the banks before; what it is doing, what's creating certain features (river features Now, and later as flows get higher and higher), look at lines, swim options, etc.
Good luck staying sane during the 'off' season!