From my swimming days i ahev developed a imbalance in strength and size between my pecs. My left pec is underdeveloped compared to my right. I have no idea how to fix this. Any ideas would be much appreciated 
Iām just curious, Did you end up swimming in a circle?
Seriously, try some unilateral work like Mike Mahler suggested here
Also do a search on T-mag and read the stuff by Ian King.