There is not really anything that can actually replace the pool but I usually recommend that people use either the row, jacobs ladder, or versa-climber machines as a substitute since they are low impact and full body workouts.

Do a couple of upstrokes to keep yourself down and it helps to do it faster. Make sure when you start your ascent you are on the bottom, so you may have to upstroke back to the bottom.

Look up co2 tables to practice breath holds. But honestly there is not a substitute for the pool.

The best way to practice is to do them. You can do some push-ups on the side of the pool till you get tired, then  dive in, do your under water and do the same thing try to monitor you interval and see how you do. I wouldn't do anything less than a 1 min interval. DO NOT DO THIS ALONE. Only practice these types of events with a buddy/lifeguard who is actively watching you and prepared to get you out and perform CPR.

You can’t really replicate pool workouts, but I would use a row machine or the versa climber/Jacobs ladder. Just something that is low impact and uses upper and lower body.

Give yourself a background in swimming. I only swam once in a summer league when I started training, but I spent 8 months prior to joining just being in the pool everyday and swimming up to 5000m at a time. Start off doing what you can and then work your way up. It is a matter of consistency and technique.

It will work, as long as you can practice swimming, treading, and underwaters. I would recommend looking up the total immersion swimming course.

For selection it will be both, for dive school it's a 12ft pool.