A 5 mm suit more than warm enough for those temperatures. In fact many people will find a 3 mm suit is adequate for those temperatures.
I encourage you to take a look at our 5 mm wetsuits. We sell wetsuits which are very popular with freedivers, but many scuba divers also use them.
Almost all freedivers use a particular "TYPE" or design of wetsuit. Why? Because it provides the most amount of warmth and the leas amount of buoyancy swing. Buoyancy swing of a suit is directly proportional to the thickness. Thick suits have more neoprene to crush so you get heavier quicker - as you descend.
Suit compression is a minor issue for scuba divers. All we need to do is press a button and the loss of lift is compensated perfectly by their BC. Freedivers have nothing to offset this swing AND their lungs are being crushed the deeper they go, so the effect is even more pronounced. Freedivers generally weight themselves so they Float easily on the surface and have to fight to get down, reach neutral at around 30 feet and from this depth onward.. they sink.. faster and faster. You can imagine that having to swim up against all this negative buoyancy (which can be over 10 lbs) with a thick suit becomes a real challenge.
This explanation is seeming to get off track, but I am trying to convey the idea about how a "freedive type" suit is the most thermally efficient design. Freedivers strive to wear the least amount of neoprene they can use - but still stay warm.
So it should not be too hard to accept the idea that a freedive suit is the most efficient design - but exactly WHY?
A freedive suit leaks almost zero water. No water will pump in and out of the suit. This makes a huge difference. How is this accomplished?
There are no zippers (to leak)
The suit has an attached (integrated) hood. So there is zero opportunity for water to leak down the neck and there is no need to make the neck seal tight or constricting. Keeping the head and neck warm is super important and avoiding cold ears makes equalization easier.
The interior of most freedive suits is smooth rubber - no nylon lining. This smooth rubber seals better than an interior with a nylon lining. Some "scuba" suits construct a wrist, neck and ankle seal with a smooth rubber collar, but a freedive suit has this type of material throughout the suit.
A freedive suit seals so well that for most people it is essential that they peal the face seal back a tiny bit and deliberately allow a little water into the hood on the initial immersion. Most nylon lined hoods leak enough on their own to obviate the need for this. A pretty clear demonstration of the relative performance of the two designs with regard to propensity to leak.
A quality freedive suit also tends to be more comfortable and flexible than a scuba suit. Why? Because a scuba suit generally uses a nylon liner AND exterior. These two layers of nylon/lycra tend to reduce the flexibility of the suit. The two layers does improve abrasion resistance and improve seam integrity, but it comes at a price. Freedive suits use an exterior nylon layer (where abrasion resistance is most necessary), but the inside is bare rubber. This design enhances flexibility of the suit. Most people call this design "open cell", although the experts in neoprene will argue that this is not the best term - nevertheless "open cell" generally is used to denote a smooth rubber (unlined) neoprene,
A freedive suit is generally quicker and easier to put on - especially with respect to thicker suit (5-7mm).
So how can it be easier to don without the benefit of zippers on the wrists, ankles and torso? The answer is that freedivers use a suit lubricant. It takes maybe two minutes to fully prepare the suit, with an internal lubricant, but then the suit slides on quickly and easily. When done properly, people are amazed.
I should mention that the smooth rubber in contact with skin and the lubricant makes the suit slide on very easily. A lubricant could be used with a nylon lined suit, but the benefit will be greatly reduced - so it is generally not done.
Also, we generally say that a 3mm open cell, two piece freedive suit (with attached hood) is equivalent to most 5 mm scuba suits with respect to warmth.
Here's a how-to video about suit lubrication and how to put it on:
