Well, people may consider doing that in the near future. Come to think of it, at least 70% of the world is ocean, and the same percentage of landmass is covered by water; that may help with the management of our ever growing population. A good idea, but we face problems.
1. Water Pressure: Like space exploration, it's pressure that can be very problematic when you have absolutely no atmospheric pressure, like in a complete vacuum or space, or when you are at our known deepest depth, 16,000 psi. (per square inch).
2. Temperature: The ocean is very deep, and can stretch down to an abyssal depth where even the sun's light can't reach. This can, and will freeze Plumbing
in future colonies.
3. Lack of knowledge/Potential Disturbance of Unknown, and Known Lifeforms:
Our race is a very messy species. Where ever we go, we leave contamination. We may accidentally kill off a very rare species of plants or animals at the bottom of the ocean that may be a valuable key to genetic research, evolutionary history, or a cure to serious diseases such as AIDS or HIV. That means, we must explore a lot more before we even set any type of foundation for an ocean colony.
4. The Sheer Cost: Unless the human race solves all it's other problems such as world hunger, or poverty, then any costly project that involves fueling an artificial habitat for humans, such as an underwater colony, or a long distance space colony, is just a pitiful attempt on trying to solve our future problems, and will cause an accumulative effect with the troubles we are going to face in the future.
(Example: A war over our dwindling land based resources)
All the problems stated have simple solutions. For example, with water pressure, look at submarines. To make a colony, we will need a shape that is resistant to pressure on all sides, and a material that can stand up to intense weights, and of course, cold.
Speaking of cold, we can solve the problem of underwater energy production easily with the abundant source of volcanic vents, or the geothermal energy they produce. This may solve electricity problems and heat problems all in one source of energy.
With the lack of knowledge problem, we should take our time and use some funds to survey what aquatic bounties we have been overlooking for centuries... Who knows how much we can learn about ourselves and the earth, just by uncovering the secrets of the ocean.
With the talk of funds... If all of us cooperate in solving the problems we are already facing, in about a decade or less, we as a species, will be able to do the impossible, (including build space or underwater colonies).
In conclusion, it takes a lot of money, thought, knowledge, time, preparation, and most especially, attention
that we do not currently give to this subject that restricts us."Its not the problems that the ocean presents to us that prevents our progression, but the problems we create for ourselves with the curiosity of much larger subjects, and most especially the thought of greed.