For active use, which inevitably means sweating, forget watertightness and seek waterproofing and breathability in worn equipment. Fabrics with waterproof, breathable membranes appear in rain jackets and trousers and footwear; at RayonRando we also offer waterproof socks and gloves, very useful for cycling, for example.
For some waterproof fabrics, durability takes priority over breathability: gaiters, subject to abrasion, and rain ponchos, which must resist tearing if snagged.
For equipment such as tents, tarps and rain covers, we want waterproofing and low weight, so outer fabrics are designed to be water-repellent as well as waterproof. A higher waterproof rating often accompanies tougher, thicker and heavier fabric.
Water repellency can be restored with a treatment, but waterproofing cannot, except by sealing holes with an adhesive patch or using silicone on seams.
Finally, RayonRando does offer equipment described as “watertight”: roll-top bags, cases and other accessories to protect your belongings. There are always limits. Likewise, some electrical, optical and electronic equipment is protected against water, sometimes carrying an ingress protection rating, such as IP57 or IPX7 for the most protective models.