Also:

"In general, the majority of horses swim well. However, when swimming

a horse for the first time give it extra attention as some horses have

been known to sink. When horses swim they employ a trotting or pacing

gait and a breathing pattern characterized by brief inspiration,

prolonged expiration, both of which look “painful and labored” to the

inexperienced onlooker. The movement of the limbs through a wide range

of motion is considered beneficial to flexibility. The difficulty in

breathing when swimming is probably due to the pressure applied to the

chest and abdomen of the horse by the water and the fact that the

horse doesn’t have the rhythm of body and abdominal movements that

serve to help the

breathing process during normal land training. As such, the horse has

to rely on the respiratory muscles and it may be that swimming is a

good way of training these muscle groups. Whether this training effect

translates to the track is unknown, but it is possible that some

direct benefit may apply. However, it should be noted that swimming

also results in relatively high blood pressures compared with

galloping and that some horses have experienced nose bleeding after a

bout(s) of swimming. As such, swimming is not recommended for horses

with respiratory disease and it is also contraindicated in horses with

back injuries."