Can I water my houseplants with sea water?

Unless your houseplants are a particular type of plant that love to be in the sea or salty water (halophytic plants), then no. Because what you're doing is adding too much salt to the plant. The roots of plants are only adapted to a certain level of salts or any kind of ions that are in the water. They're only able to take up water in a certain concentration. If that becomes more concentrated the osmotic pressures go crazy and the plant is unable to take up any more water. So unless you are growing seaweeds, sea grasses or salt marsh plants then I think, stick to fresh water!