## 1 Introduction  
Swimming is a sport, like many others, where time over a given distance (i.e., speed) is the main performance indicator. During training sessions, various speed-related parameters, such as race pace, maximal aerobic velocity, critical speed, and others, are used for monitoring training intensity [1, 2]. However, when it comes to monitoring or evaluating training intensity in real time, land-based sports can rely on a larger variety of physiological or biomechanical parameters. This is due to the larger spectrum of wearable technologies available for such purposes (for example, training load by GPS, oxygen consumption by spiroergometers, etc. ), resulting in numerous ways of in situ or a posteriori monitoring of physical effort during sports training or even during competition. Portable heart rate (HR) or oxygen consumption (VO\({}_{2}\)) measuring devices can be worn in ecological scenarios for a live assessment of the physiological demands of many activities, such as running [3], cycling [4], tennis [5; 6], or soccer [7]. For HR measurement, on land one can rely on valid and reliable HR chest straps that have been used for decades and are deemed the portable HR measurement gold standard [8; 9; 10]. While these chest straps use electrocardiography measures (i.e., they capture the electrical activity of the heart to detect the QRS complex), other portable sensors, such as built-in wristwatch sensors and other multi-site sensors, use photoplethysmography (PPG). This technology analyzes the subtle light intensity shifts in the arteries caused by changes in blood volume, which are also felt as pulsating blood. Although the measurement principles are completely different, these devices have shown relative robustness in measuring HR [11; 12]. However, these assessments of measurement agreement take place almost exclusively in terrestrial environments, leaving it unclear whether the same technology would be equally accurate in aquatic environments. In water-based environments, monitoring training or competition performance live based on popular physiological parameters such as HR or VO\({}_{2}\) becomes impractical. Not only it is essential to overcome the obvious incompatibility between electronic components and water, but there is also the problem of data transmission. Almost all wearable devices transmit to a receiver using technologies such as Bluetooth or ANT+, which, despite their advantages, do not function in aquatic environments. That's why today, some sensors offer built-in memory, so swimmers can save their training effort and analyze it afterwards. Nevertheless, the saved values still need to be accurate. The body displacement through the water during actual swimming can lead to water passing between the PPG sensors and the skin, potentially impairing the readings [13]. Studies have shown that the pressure between the PPG sensor and the skin influences the reading accuracy, thus the sensor cannot be worn too loosely or tightened to the point of discomfort but requires a moderate pressure, highlighting the importance of full and constant skin contact [14]. Even chest straps might have trouble getting accurate readings during swimming due to the increased water drag after the start and the push-out off the wall after each turn, which might cause the chest straps to fall out of place. Swimmers could bypass this issue by wearing a swimsuit, but this might alter their swimming mechanics and/or buoyancy. Recently, an alternative approach was developed for one of the PPG sensors available on the market. This specific sensor (OH1+ and Verity sense, Polar Electro, Finland) has been provided with a clip that allows it to be attached to the swimming goggles strap, in this case, and to read HR from the temple. Given the placement site, the sensor can be covered with a swimming cap, which, in theory, might overcome the difficulties mentioned above. Nevertheless, it is still an a posteriori approach since the sensor saves the data to be analyzed later.