## 1 Introduction  
Cardiovascular diseases constitute the second most common cause of diving fatalities [1]. The Divers Alert Network (DAN) presented in their report some scuba diving fatalities involving U.S. citizens, indicating cardiac diseases as an important factor for those incidents. In many cases, autopsies could not determine people's cause of death. It is assumed that they have died as a result of cardiac dysfunction, because they had manifested coronary artery diseases (CAD). However, the observed lesions have often not been significant enough to warrant an intervention and the deceased had no reported issues prior to the event. Most of these cases are'silent' with no prior indication of a problem. It seems that in an immersed condition, minor cardiac dysfunctions may quickly lead to major problems for divers and especially snorkelers [2]. It is known that heart rate variability (HRV) is high at rest and decreases during physical activity [3]. Either a lower than normal HRV under controlled conditions or a reduction in HRV induced by diving would be pathological. It was proposed that a scuba diving candidate showing such characteristics should be further investigated [4]. Therefore, HRV is a highly relevant measure for assessing the autonomous nervous system and has been used for analyzing stress [5], sudden cardiac death [6], or Parkinson's Disease [7]. The pattern of fatality indicates that there is a need for cardiac monitoring while snorkeling or diving in order to predict abnormal cardiac activity during immersion and subsequently activate preventive measures [8]. However, this might not only be important for underwater sports, but also for the monitoring and early detection of heart diseases in elderly [9] during recreational or sporty water activities. It seems plausible that such a monitoring system must be able to correctly assess the HRV as a very important measure of possible heart dysfunction during the transition to a submerged condition. Cardiac activity assessment can be realized using electrocardiography (ECG). There is a wide range of ECG measurement devices for dry conditions, e.g., classical adhesive electrode based solutions, shirts with integrated textile electrodes, handheld devices with metal plate-based electrodes or even chairs with integrated electrodes [9]. Previous studies have also demonstrated the possibility to apply ECG monitoring technology underwater. [PERSON] [4] performed a study to determine the HRV of divers. [PERSON] et al. [10] presented a study using a 12-lead Holter monitor to record divers' ECGs. [PERSON] et al. [11] developed a mobile phone-based ECG, building on the work of [PERSON] et al. [12], for real-time monitoring of scuba divers. All these systems are based on the concept of potential measurements on the skin surface. However, during immersion in water, this surface potential becomes nearly equipotential due to the high conductivity of water. To allow such ECG systems to still operate underwater, the conventional technologies require adhesive, water-tight sealing of the electrode-skin interface or a complete water-resistant wearable sensor design [13]. Special electrodes employed in underwater potential measurements without adhesive waterproofing were proposed by [PERSON] et al. [14] and [PERSON] et al. [15]. While [PERSON] employed an electrode coated with non-conductive material including a high-input impedance amplifier, [PERSON] designed an electrode containing two hydrophobic properties, Polydimethylsiloxane (PDMS) and Carbon Black (CB). Pilot studies showed promising correlation of ECGs measured in dry and immersed conditions [16; 17]. However, these methods are costly, require an increased manufacturing effort and decrease in performance over exposure time. Furthermore, actual proof of functionality is still missing, since these electrodes were only tested in a bathtub rather than in scuba diving scenarios.