I'm surprised Vince isn't all over this... like this one: http://community.magicseaweed.com/viewtopic.php?f=4&t=37026
I vouch on everything what Gav said. Brilliant suggestions and great link for back stretches/movements. These are very common exercises that physios, osteos, and chiros may suggest to people with back problems. Not all these exercises on the link applies to every back pain though. So try one or two, and see how your back feels. If any of the movement improves your back movements and less pain, spot on. If it's painful, then stop (obviously...), try something else.
iionzii wrote:I feel like my spine bone has actually curved due to paddling on the board
Do you mean lordosis? A common posture for surfers...it's not a condition or bad thing, it's just how people are. Some have really arched back (lordosis), some have really flat back, sway back, hunchback...everyone's different. If there's pain associated with the posture (like yourself?), then it needs managing. Try physio (I confess that I am biased) for just an assessment.
In terms of swimming, two little suggestions might come in handy:
1) try to include backstroke in your session (alternating with front-crawl ever other length during warm-up / cool-down is a perfect start). It adds a good mix to your overall shoulder movements, and backstrokes help to open up your chest muscle groups (makes your back and chest muscle groups balanced and supple). Plus you it's bit of a change from staring the dull line on the bottom of the pool...
2) At the moment your total distance in one session is 1,200m (3 blocks of 16 x 25m). Keep this total distance for now, and once you are competent with sets of 25s, try to build up to a full 400m on your main set. Take your time over many sessions, gradual increment on your main set (for example, 8x50s 20sec rest, 4x100s 60sec rest, 2x150s 60sec rest + 1x 100, 2x200s 90sec rest, 300m + 100m, then finally a straight 400m) will prevent the pitfall of "training too much too quick" and will avoid over-training injury. This might help you with building your strength and stamina to swim multiple 400s, then rest is what Gav mentioned above.
In swimming, 400m is something equivalent to 1mile run on a track. Too short distance to "pace" like a marathon, but too long to go all out from the start like 100m sprint. It's the most challenging distance in "competitive" swimming (I think). It's a good baseline measurement to take, time yourself once in a while, and see how much you progresses over time.