Some Seacock 101, What is a marine seacock, some tips on replacing them, what you will need, and some background info. Part 2 will include on the boat removal and installation tips.
Parts used in the video can be found here. These may not be the best prices. I'm just providing links to the products I used.
Backing plate: https://www.amazon.com/gp/product/B00TUNZ8G4/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
Seacock: https://www.westmarine.com/buy/apollo-valves--full-port-flanged-bronze-seacocks--P011_332_003_006?recordNum=1
Mushroom: https://www.westmarine.com/buy/groco--3-4-thru-hull-fitting--257274?recordNum=4
90° hose fitting: https://www.westmarine.com/buy/groco--full-standard-flow-tailpieces--P011_332_001_025?recordNum=8
Thickened Epoxy Six10: https://www.amazon.com/West-Systems-610-Hardener-Cartridge/dp/B002IVAJ6Y/ref=sr_1_1?keywords=six10&qid=1557991122&s=gateway&sr=8-1
Adhesive/sealnt 3m 4200: https://www.amazon.com/3M-Marine-Adhesive-Sealant-Gallon/dp/B001QWN8VE/ref=sr_1_4?keywords=3m+4200&qid=1557991159&s=gateway&sr=8-4