Biocompatible Hydrogels, which are essentially tough water, were established in 1960 by the Czech scientists Otto Wichterle and Drahoslav Lim as novel material for soft contact lenses. Their contribution to life science opened new frontiers in research including tissue engineering and smart drug delivery systems with hydrogels. Even though the first application of hydrogels were soft contact lenses, little attention has been paid using hydrogels for optics since then. Nevertheless, hydrogels show a large potential in biocompatible tough water optics, designed to work inside the human body as either sensor or soft machine. Hydrogel-based sensors or prosthetics - such as an optical waveguide or an engineered eyeball - need soft and transparent materials that provide additional functionality, such as light guiding capabilities or an adaptive focus. Here I demonstrate a stretchable step index fibre with a core diameter of 340 and 780 m, capable of transmitting light over a length of 10 cm, completely made of hydrogels. Instead of combining different hydrogels, I tune the gels mechanical and optical properties, like modulus of elasticity - from 0.5 MPa down to 0.05 MPa - or refractive index - between 1.426 and 1.45 -, directly during the polymerization process. To do so, an in-depth analysis of the material properties and its swelling behaviour is provided, including experimental data and theoretical modelling. The novel semi-analytical approach leads to a description of the physical crosslink density, in the hydrogel, as a function of the polymerization conditions. The developed methods enable rapid prototyping of soft, biocompatible optical devices and sensors, that may find application in wearable healthcare devices, such as continuous glucose monitoring.