Theres alot of good info on the forum and the 'net on fitness training.
However, you might be referring to the CF Express test and the
physical requirements of BMQ.
Consider in BMQ, you'll be marching, climbing and moving around obstacles,
practicing drill, orienteering or crawling over terrain, carrying loads, and
it takes a combination of cardio-respitory fitness, strength, and endurance.
Any combination of exercises whether its swimming, running, or whatever
that develop these 3 things will be benefitical. An example of this
is a 13 km march near the end of BMQ; the recruit carries a weighted ruck-sack
and fast marches for 13 kms. It takes physical conditioning to achieve it.
Personally, I find running the best for developing cardio-resp. A good
overall-body weight lifting program for muscle strength. For muscle endurance
any fast sports, power walking, jogging for distance, swimming, cross-training,
are examples.
Swimming is good for endurance but its better to mix a number of activities
to get the benefits of all.
Its important to train smart. You want to ramp up your fitness level but not to
the point of injury. Avoid making common training injuries worse by over-training
or over-exceeding your limits.