Colour fades faster in this country than the instructions on any box assume, and three things do most of it: sun, salt water and hot showers.
What genuinely helps is unglamorous. Wash in water that is cooler than feels nice. Rinse with fresh water immediately after the sea, before the salt dries. Keep the first wash at least two days after colouring. And cover your head in the middle of the day, which does more than any bottle.
Products do help at the edges. A sulphate free wash is gentler on colour than a strong cleansing one. A weekly mask keeps the ends from going porous, and porous ends are where colour escapes first.
What does not help, as far as we can tell in this room, is most of what is sold as colour locking. We stock two of those and we will tell you plainly that the difference they make is small compared to the shower temperature.