Here's an educated guess:
The Navy physical readiness requirements for running is a 1.5 mile run. For swimming it is either a 500-yd or 450-m swim. I guess that means the Navy roughly equates the two interms of physical readiness.
SO I am going to assume you run 5-6 miles an hour ( a nice average/slow pace). That means in a half hour, you run 2.5-3 miles. That means roughly a 675-900 meter (13.5-18 laps) swim.
The logic seems to work, and 13-18 laps of swimming will give you a pretty good to pretty hard workout, if you are a non-avid swimmer like myself.
I pretty much agree with you (having taken the PRT myself), especially for a newer swimmer. Just remember that humans are built for the land... as you get a better feel for the water, you should be able to do much more swimming than the amount you wrote.
Also, I think this has been beaten to death on this board, but, in the swimming world, I have never heard anyone consider a lap to be more than one length of the pool. In fact, I think I may go back to that other post and mention it.
Well said, sladekos.
I think that the previous post was a good starting number at about 900 yards in a half hour. As you become more efficient in the water, you should be able to increase that number to 1000-2000 yards and up, depending on how serious you want to get. The important thing to note is that swimming is low impact. The body takes a beating in a half hour of running. As you get in better shape, you may be able to easily swim 1-2 hours without the discomfort associated with pavement impact during running.
There was an article a while back from Slowtwitch that broke down workouts into points:
1 Point for 1/4 Mile Run
1 Point for 100 Meter Swim
1 Point for 1 Mile Bike
No points for any other workouts like Yoga, Weights etc.
It sounded good to me at the time because it was based on the Distance comparisons.
I can swim at a rate of about 1 meter per second for 1.5 KM and so it takes me 100 seconds to score a point in the pool. (Or 15 Points in 25 Minutes)
I can run 1/4 mile in 2 minutes paced for an 8 mile run or 32 points in an hour.
I can ride 40 KM on my bike (Just under 25 miles) in 75 minutes for a total of 25 points.
These all feel about the same at the end assuming roughly the same level of effort, but that may be because I have been using this method for a while.
The article is still available in the archives someplace.