Depends on what kind of wetsuit you buy.
Wetsuits work by trapping a layer of water against your skin which is then warmed by your body, and insulating this warm water from the cold water outside the suit, slowing down the rate at which your body loses heat to the water. A wetsuit will only keep you warm if it covers sufficient skin with sufficient neoprene (relative to the water temperature) that the rate of conductive heat loss through the suit is equalled (or exceeded) by your rate of metabolic heat production. It also needs to fit well, because a loose suit will allow cold water to flush through it, displacing the warmed water.
For water temps. of 40-50°F (5-10°C), a basic 1-piece 'steamer' suit thick enough to keep you warm while exercising (even taking into account the fact that metabolic heat production will be raised) will likely not be comfortable/easy to swim in, since it would have to be so thick (4-5 mm) as to restrict your range of arm motion.
Your best option would probably be to look at wetsuits designed specifically for surfing or triathlons. These are generally made of more flexible neoprene than basic wetsuits, which allows greater freedom of movement, and also include taping over the internal seams, which increases seam strength while reducing flushing. These suits also generally have thicker neoprene (5-6 mm) on the torso and upper legs, and thinner neoprene (2-3 mm) for the rest. As you might expect though, the degree of suit engineering necessarily increases the price.
Failing that, you might also consider buying a sleeveless 'farmer John' style wetsuit, plus a thin (2-3 mm) shortie over the top to 'seal' the gaps under your arms which would otherwise allow (cold) water to flush through the suit from armpit to ankle while you swim. The seams around the collar and upper arm cuffs of the shortie would themselves have to be wide/ tight enough to prevent flushing.
Hope this helps.