In ectotherms, thermal acclimation and behavioural thermoregulation have evolved to match organismal performance with local or temporary thermal conditions. In semi-aquatic species, however, this matching encompasses a trade-off: organisms that thermoregulate close to optimal muscle function on land will inevitably depart from that optimum when entering water, a medium that may differ drastically in temperature. With regard to predator evasion and foraging success, how do semi-aquatic ectotherms deal with such a challenge? We experimentally raised young semi-aquatic Tiger snakes in either terrestrial or semi-aquatic environments over 11 months. When tested in a standardised enclosure, young snakes raised in a semi-aquatic environment selected slightly, but significantly higher mean body temperatures than their terrestrially raised siblings (respectively 30.3°C versus 29.5°C). The former allowed their body temperature to remain higher than 32°C for twice as long as the latter group (4.4 hours vs 2.1 hours). Locomotor performances (swimming speed) were, unsurprisingly, strongly linked to body temperature. Entering water with a higher body temperature (30°C versus 19°C) delayed a sharp drop in locomotor performances, and thus lengthened maximum performance time. We hypothesise that young snakes, by allowing their body temperature to reach above their usual optimum body temperature, may delay the drop in locomotor efficiency in case of foraging opportunity or in order to escape a predator.