In surfing or whitewater where intensity is up you may be too tense while paddling. Loosen your paddle grip, relax, don't paddle too hard into waves, take less waves, maybe carve more rather than use the paddle as a pivot or brace.
Last weekend, I began to use a technique i used in surf kayaking where i'd literally slide into a wave with as few if any strokes as possible, therefore reducing the quick cadence i normall used to get in.
also are you using your torso/core for power or just arms? think about a straighter arm(s) stroke, but a loose grip, let the body do the work not the arms or shoulders alone.
many prefer a shorter paddle in surfing. i use my regular flat water length which gives me more power and bracing support, plus keep my back straighter.
also look into yoga and/or find a very good masseuse to work on those areas.
Classic Freestanding Rack