Swimming

Someone I know went from no swimming (enough to not drown) to full Ironman (Weymouth) in similar timescales.

He had weekly 1:1 lessons in a pool and one or two extra sessions on his own each week to build up his stamina once he had got the basic technique sorted. Near the end he did some open water (lake) swimming to get used to that. Not sure if he did any sea swimming before the big event, will ask next time I see him.

Open water swimming in a lake is quite different to a pool as you get no mini rests at the end of each length, sounds obvious but it takes a bit of time to adjust your pace to be used to it. Then you've got the added fun of sighting and swim biff.

Open water sea swimming is then another chunk of different on top. Salty water, swell/waves, currents, jellyfish (ugh), etc.

Hoping I'll be reasonably safe if I get myself a thick wetsuit

Most swimming wetsuits are the same thickness, too thick and it's hard work to swim in. Many open water swimming places have rentals and then ex-rental or stock clearance sales.

an inflatable thingy that you tow behind you

A tow float, mostly for visibility rather than emergency flotation, but also useful for carrying stuff in you get a dry-bag version. Never used one myself.

and like a whistle or some shit for emergencies.

Attach one to the tow float if you want. I've only swum where there are people looking out for me (kayakers patrolling the lake, or lifeguards on the shore).

Where are you based? People here might have some recommendations for open water swimming...

Better to get better in a pool first before venturing outside. As @Greenbank the next logical step is an organised lake where there are lifeguards and stuff, plenty around the west side of London but most will have closed for the winter now.

If you're really keen on sea swimming look up your local open water swimming society, usually found on Facebook and there are likely to be crazy regulars there who are super encouraging of new people to the sport and probably swim all winter in skins (no wetsuit).

swimming on your own is really not recommended, between just getting into difficulty, currents or water craft can all go from something minor to a full on disaster very quickly. whistle or no whistle if there's no one watching then it ain't gonna do much good!

Tow floats are good for visibility and a bit of buoyancy, but I would want to need one if you know what I mean, they do all come with whistles but as said before who's attention are you trying to get, and if they heard you blowing away are they going to be able to do anything to help you in time!

Wetsuits are definitely a good idea for beginners both for floatation and warmth when you are first going from a 23 degree pool to 10 degree water it can be a bit of a shock!

Finally, MP3 players in the pool are a bad idea as you can't hear other things around you but are an absolute no no in open water, if you can't hear the boat coming towards you how are you going to get out the way!!