Summary - While Sarracenia need to be kept wet in cultivation, saturated and stagnant conditions can be dangerous for general plant health. Sit Sarracenia in a tray of water about 5 cm (2") of water if you are using conventional pots. Deeper water can safely be used with mesh or basket type pots and is definitely needed if you use Sphagnum. It may be possible to overcome hard water if you use a commercial aquaraium water designed for use with Amazonian fish like Discus, but this needs investigation.
Water is essential for all living things for a variety of reasons. In plants, it is used to carry energy derived from photosynthesis (glucose and other sugars) and minerals throughout the plant, as well as keeping the leaves and other structures firm.
Because Sarracenia are bog plants, they live in areas with naturally high water tables. In many cases, the rhizomes of the plants are actually underwater. Obviously, then, they need lots of water in cultivation.
Because we are holding our plants in a container, there are some modifications that need to be made. If you use a solid-walled pot, you can't sit the plant completely under water. Why? Because the roots won't be able to get enough oxygen. In the wild, plants with roots under water spread them out over a relatively large area. Oxygen from the air penetrates the water and soil over all of this area, giving the roots enough oxygen to breathe. In a pot, the area of oxygen exhange is limited to the surface of the pot and those tiny holes at the bottom. This is a lot less surface area, and therefore a lot less oxygen to get to the roots. In addition, the roots of plants in smaller pots tend to get very potbound, meaning there is a lot more root competing from a limited amount of oxygen. For this reason, I always sit my potted plants in no more than 5 cm of water for a standard height, 20 cm (8") pot. This leaves most of the soil above water and able to breathe more than if it were underwater. The compost still stays wet, because peat is great at absorbing water and holding it. I am yet to have a plant die from dessication when grown in this way.
Basket pots with mesh sides let in a lot more oxygen, which is both a plus and negative with regards growth. If the plant is not wet enough, too much oxygen gets to the roots and can dry them out or, at best, severely restrict their growth. As these are best used with Sphagnum, they need a permanantly high water table that largely solves this problem. Peat-based mixes are better suited to standard pots anyway, so the extra expense of basket pots generally negates any benefit. However, it may be rewarding to experiment with cutting small slits or holes
For acid loving plants like Sarracenia, it is also important that the water you give them is acid and soft, because hard or alkaline water will literally burn their roots and kill them. The reason for this is simple - water with a neutral or low acid present will untie all the minerals in the soil that are usually tied up when lots of acids are present. The abundance of these newly liberated minerals bombard the roots, which contain few of these minerals. The damage caused by this bombardment causes the plant to expell water in a vain attempt to wash the chemicals away, 'burning' the roots to death. The plant then wilts and dies.
In Australia, tap water in major eastern cities is generally ok for Sarracenia, but will eventually deplete the acids present in peath moss and Sphagnum. Repotting with fresh peat is then needed to restore the acid conditions. Where I am in Canberra, the tap water is soft and acid enough that peat lasts much longer. In fact, the water here is so soft that it can be necessary to add salts to the water if you want to keep Amazonian aquarium fish like discus (pet store owner, pers. comm.). This is astounding, because in most parts of the world, people use water conditioners to soften the water, not harden it with even more salt.
This gave me a brainwave. Theoretically, it should be possible to soften water in an appropriate manner for Sarracenia by using a water conditioner for discus. I have never tried this or used the water conditioners myself on aquarium fish or plants. But given discus fish need to live in soft, acid water because of their osmotic balance requirements, the conditioners do seem to be relevant and applicable here. I also hasten to add that the discus water conditoners are not the same as the commonly available chlorine neutralisers or pond blocks - these actaully add even more salt and make acid water harder and less acid. The type of conditioner I am referring to is very specialist and available off the web or from aquarium shops. If you want try it, tell the sales person you want something to prepare water for discus fish, and they should point you in the right direction. Just make sure the product you buy actually specifies that it both acidifies and softens water.
I'd be interested to hear success or failure of this. I've never tried it, because I have never had need to. It just seems logial to me. If you try it, use something expendable before trying it on your best plants!