Larger areas of continental part of Croatia do not meet the standards prescribed by „World Aquatics“ in regard to pool measurements. When it comes to competitive swimming, the lack of adequete conditions is a major issue. Consequence of aforementioned circumstances causes swim training to be applied in thermal waters, which can potentially affect the competition performance of swimmers . Therefore, the aim of this research was to determine the differences in effectiveness in swimming performance and workload with respect to water temperature. Testing protocol included 11 swimmers (6 male, 5 female), who did the critical swimming speed test (200 meters and 400 meters), front crawl at maximum speed, first in thermal, than in cold water. Timing of swimming on both 200 metres and 400 metres length using crawl technique at a maximum speed was measured at the end of swimming sections. After completion of swimming protocol, the data regarding the lactates and heart rate was collected. The heart rate data was collected using the palpatory method (radial artery). The statistical analysis of the data revealed the difference in swimmers’ heart rate after swimming 200 metres front crawl at maximum speed in both thermal and cold water. Statistically significant differences in heart values were 145,45±21,66 beats per minute in thermal water, 131±14,95 beats per minute in cold water (p=0,03). Other results did not show a statistically significant difference. Although the differences in the mean of the critical swimming speed did not show a statistically significant difference (p=0,73), it amounted to a 2 second difference per 100 metres, it represents a big difference in practice.