I'm not going to venture to give investment advice (don't have enough experience of it yet), but I've invested in both of these sectors using Exchange Traded Funds. They can be bought and sold as shares on the London Stock Exchange. You can even put them in an ISA to make sure they're capital gains tax free:
Water:
iShares S&P Global Water (IH2O) - this invests in the S&P Global Water index, an index of the top 30 listed water-related companies.
Advantages: wide geographical spread, blue-chip companies, steady income even in downturn
Disadvantages: equities are quite high at present so may be due for a fall, might miss out on 'takeover' premiums as the companies are all so large
Gold:
ETF Securities physical precious metals basket (PHPM) - this invests in gold, silver, palladium and platinum. EFT securities also have a gold-only fund (PHAU).
Advantages: low management fee of 0.39% PA, don't have to store the gold yourself, easier to buy and sell than coins
Disadvantages: gold is very volatile, if you really think the global financial system is going to collapse you might want to buy physical gold coins etc for yourself