For those not familiar with what constitutes a swim check, the explanation used on the Longhorn Council website for Sid Richardson Scout Ranch is below. If you want more information, see this PDF document.
Classification of Swimming Ability
The Swimmer Test
The swimmer test demonstrates the minimum level of swimming ability required for safe deep-water swimming. The various components of the test evaluate the several skills essential to this minimum level of swimming ability: Jump feet first into water over the head in depth, level off, and begin swimming. Swim 75 yards in a strong manner using one or more of the following strokes: sidestroke, breaststroke, trudgen, or crawl; then swim 25 yards using an easy, resting backstroke. The 100 yards must be completed in one swim without stops and must include at least one sharp turn. After completing the swim, rest by floating.
The test administrator must objectively evaluate the individual performance of the test, and in so doing should keep in mind the purpose of each test element.
"Jump feet first into water over the head in depth, level off, and begin swimming. . . ."
The swimmer must be able to make an abrupt entry into deep water and begin swimming without any aids. Walking in from shallow water, easing in from the edge or down a ladder, pushing off from side or bottom, or gaining forward momentum by diving do not satisfy this requirement.
". . . Swim 75 yards in a strong manner using one or more of the following strokes: sidestroke, breaststroke, trudgen, or crawl; . . ."
The swimmer must be able to cover distance with a strong, confident stroke. The 75 yards must not be the outer limit of the swimmer's ability; completion of the distance should give evidence of sufficient stamina to avoid undue risks. Dog-paddling and strokes repeatedly interrupted and restarted are not sufficient; underwater swimming is not permitted. The itemized strokes are inclusive. Any strong side or breaststroke or any strong overarm stroke (including the back crawl) is acceptable.
". . . swim 25 yards using an easy, resting backstroke . . ."
The swimmer must indicate the ability to execute a restful, free-breathing backstroke that can be used to avoid exhaustion during swimming activity. This element of the test necessarily follows the more strenuous swimming activity to show that the swimmer is, in fact, able to use the backstroke as a relief from exertion. The change of stroke must be accomplished in deep water without any push-off or other aid. Any variation of the elementary may suffice if it clearly provides opportunity for the swimmer to rest and regain wind.
". . . The 100 yards must be swum continuously and include at least one sharp turn. . . ."
The total distance is to be covered without rest stops. The sharp turn simply demonstrates the swimmer's ability to reverse direction in deep water without assistance or push-off from side or bottom.
". . . After completing the swim, rest by floating."
This critically important component of the test evaluates the swimmer's ability to maintain in the water indefinitely even though exhausted or otherwise unable to continue swimming. Treading water or swimming in place will further tire the swimmer and are therefore unacceptable. The duration of the float test is not significant, except that it must be long enough for the test administrator to determine that the swimmer is, in fact, resting and could likely continue to do so for a prolonged time. The drown proofing technique may be sufficient if clearly restful, but it is not preferred. If the test is completed except for the floating requirement, the swimmer may be retested on the floating only (after instruction) provided that the test administrator is confident that the swimmer can initiate the float when exhausted.
Reference: Swimming and Lifesaving merit badge pamphlets
Jump feet first into water over the head in depth, level off, swim 25 feet on the surface, stop, turn sharply, resume swimming as before, and return to starting place. The entry and turn serve the same purpose as in the swimmer test. The swimming can be done with any stroke, but no underwater swimming is permitted. The stop assures that the swimmer can regain a stroke if it is interrupted. The test demonstrates that the beginning swimmer is ready to learn deepwater skills and has the minimum ability required for safe swimming in a confined area in which shallow water, sides, or other support is less than 25 feet from any point in the water.