Low air temperature - should I use a wetsuit in training?

In this week, the air is 16°C and the pool is 20°C at my location, and it is the perfect temperature for me to swim in without any wetsuit. (I feels heat neutral in around 21°C, when the water is 24°C I begin to feel a bit warm after doing high-intensity swimming) However, the weather forecast is predicting low air temperature - around 12°C next week. The pool I'm using is a 25 m outdoor pool with water depth 1.2 m - 3.0 m end to end. I don't know how fast the pool cools down, so I have a few questions: How is the feeling different between 12°C air and 18°C air, when the water stays constant at 18°C? In particular, is "the average of air and water temperatures" a good indicator? I have never swum seriously in my wetsuit before - I primarily use it for sailing in cold weather. I think it may be very bulky and uncomfortable to swim in, is it true or not? I've heard that the speed will be faster when swimming in a wetsuit - especially when doing triathlon - is it true? Although in my area we don't have triathlons in wetsuit legal season so this is temporary a non-issue for me. My endurance target is to swim for 2 hours in 15°C water this year - and for 6 hours next year without wetsuit (I'm talking about normal conditions - not something like 3°C air and 15°C water as happened 3 years ago! That would be crazy!). However, I also have to build up my speed as well. I'm trying to use various toys to see if there is a particular dead spot, but I tried using fins, snorkels, paddles and pull buoy one by one, and they all provided no difference in my speed and efficiency with or without them when I tried. So the next thing I may try is putting my wetsuit on - but only when it is cold enough! When doing technique sets or speed sets in cold weather, in order for the training to be more effective, should I put on my wetsuit or not? (an extreme case will be 6°C air and 13°C water - is it foolish to do a standard 45 minutes training, consisting of drills and intervals, on bare skin?)