Aquaculture is presented as a sustainable method to provide fish, although in reality, it is far from being sustainable. Its negative impacts on the environment can be prevented and corrected by the use of sensors, developing precision aquaculture. Sensors are widely used in terrestrial applications, but in underwater environments, their use is constrained by a variety of issues. The aim of this paper is to describe the state-of-art of the underwater sensors for water quality monitoring. First, the requirements and challenges of underwater sensors for aquaculture monitoring are discussed in detail. The main challenges are the need of a waterproof isolation or the need to avoid corrosion and biofouling, among others. Second, there are some advantages compared with terrestrial applications, such as no need of minimized systems or the fact that such systems only require low accuracy. Subsequently, we evaluated the different options available to sense each variable, related to the needs of the aquaculture sensors. For temperature monitoring, thermistors, thermocouples or RTC seem to offer similar advantages. In contrast, for dissolved oxygen monitoring, the optical method seems to be the best option. For turbidity, optical methods are the most employed ones, while for conductivity measurements, the inductive coils are a promising option.