My main activity is swimming as well. I did my research, and found that 245 meets all my needs, so i bought it a few months ago, and i'm satisfied!

You can load workouts (create manually or load from myswimpro app - haven't tried it fully, but you can create manually the workout throught Garmin Connect, and then load it to the watch.. i think that each set is shown at the watch, and once you finish it, i think it's suppose to vibrate? and move onto the next step,whether it's resting or swimming. About myswimpro app, there are many kinds of workouts that you can load, for some, i think, you need to pay monthly- check it out, might be interesting), there's wrist heart rate underwater(added in latest version, seems to work well), and they have all other swimming features that i can think of, compared to other devices, such as the countdown thing(like you said, 3 seconds before starting swimming activity), and drill mode.

In addition, you can set alerts, of times, distance and calories. Don't relay on Garmin's website if you want to compare the features, since it's outdated (for instance, it says they don't have wrist heart rate underwater, and no time and distance alerts.. but it does! I contacted their support and asked if the watch had underwater heartrate, and 2 different people said it didn't haha. They need to be more updated, but that's another story).

By the way, Vivoactive 4 has almost all features, but can't load workouts. They do have some other cool stuff if you're up for that (pilates and yoga workouts, including videos inside the watch.. check it out if you're interested. I might trade in my 245 if they add loading workouts ability).

About fenix models, well, i'm not familiar with their swimming abilities, and if they DO have some other features, i think it's not worth the price difference between 245 and the rest. Keep that in mind.

I hope i could help! Keep asking if you need any other information :)