This is what I do for a constant supply of watercress from mid April through to Autumn: -
Sow the seed in a half sized tray and cover with a little sifted compost. The seed is very small and needs to be spread as evenly as possible. Water or stand the tray in water until the compost is damp. Keep in a propagator or in a plastic bag and place in a warm place. Treat as you would any other seeds keeping the compost from drying out. The seeds will germinate in about 10 days after which they need to be kept in a well lit position (not direct sunshine) or the plants will go leggy. If the seeds are in a cool place the germination rate will be poor and also if its too cool for the seedlings growth will be slow (same as most seeds / seedlings at this time of year).
Once the seedlings are about 1" tall prick out small clumps of them into 10"- 12" diameter plant pots containing 75% multipurpose compost and 25% sharp sand. Better to soak the compost prior to pricking out the seedlings as the seedlings will still be small at this sage and are easily flattened when watered. I am guessing here but I would estimate about 40 - 50 seedlings per pot.
Place the plant pots into a large watertight container (sink) and then fill the container to about 1/2 to 3/4's way up the side of the plant pots. If you can, keep the container in a warm, shady place i.e. greenhouse, garden frame until it warms up outside when it can be placed outside again in a shady place. I just keep it underneath the greenhouse bench in early spring. The water only needs changing every few weeks to prevent it going stagnant. Whether it makes any difference or not I don't know but I use rain water in the container.
Once the watercress is of a suitable size just cut as required leaving a few bottom leaves on each stem in place and it will continue to grow until the nutrients are used up in the compost after which growth will slow. To overcome this either sow in succession every 4-6 weeks or plant rooted cuttings in new compost.
1 or 2 packets of seeds will keep you going all summer. I have tried seeds from both Marshall'S and Thompson & Morgan with equal success.
Be careful where you place the container as like most salad crops slugs and snails can be a nuisance.
Good Luck