Hi Mate,
I used to sell Nikwax and Fabsil in my previous job and found Nikwax to be the better. However washing any waterproof item in a normal detergent will rip any water repellency from the surface including the original factory applied repellency. You can either use a special detergent for washing waterproof items (the green nikwax bottle) or good old soap flakes! All these "reproofers" are just a water repellency so dont actually make the garment water proof again. I found spraying an item then saoking a sponge in Nikwax and going over all the seams thoroughly was the best way to get the best result. Salt water also damages any applied repellency so they wont last long if used regularly. Also if its a breathably garment use a spray instead of a "wash-in" as the "wash in" will apply to reproofer to the inside of the garment and repel your sweat, keeping it trapped in your clothes.
Hope this helps haha!