Its easier to lift things underwater. :-)

You can do both the same day. If you get cold after a swim, as I do, you may still need a warm-up. I'm not sure I would swim after lifting - muscles might lock up. Split training would be fine and swimming 5 days a week is good too. Swimming burns more calories than almost any other cardio workout, so if you are swimming/lifting back-to-back make sure you consume carbs an hour before your workout.

It entirely depends on the level of intensity and duration of your resistance training, and also how high and how long you are going to increase your heart rate during swimming. Cardio can cost you size if it's too high impact, and can be less than beneficial if it's too low impact, or if it is for too short a duration. If you are lifting heavy and want to preserve those gains, swim for 30 minutes at a time, getting your heart rate to between 70 and 75 % of your target heart rate (i.e. if you're 40, then 220 - 40 = 180 X 70% = 126). There are more pinpointed ways to calculate your target zone, but this method will suffice for most people.

Remember, in order to start burning fat, you need to be in target zone for 15 to 18 minutes first! Then, and only then, do you start burning fat. That's also where your lifting program is key to this. Resistance training comes in wide variety, from limited cardio to maximal cardio benefits. Rolling right into swimming could keep you within target HR the whole time, but often it will not.

Good luck!
Jeff Gray
AFAA Certified Trainer
Owner - The Gray Area Fitness and Form Training
626.643.7950

Good reply from 'Ever' to your last question. Swimming, for what you describe, is 90% about cardio, and maybe 10% muscular development. Give your major, larger, higher range of motion muscle groups (chest, legs, shoulders, back) 2 days rest, and smaller, limited range of motion muscle groups (arms, abs, even calves can fit in here) 1 day rest. If you are ever to the point of considering competition, that's when yo would need specific tweaks, and more carefully monitored CV training.