They over come this problem in several ways

1. They have root cells which concentrate a lot of salts in them and this enables to take water by osmosis in the normal way.
2. Some plants have salt glands that secretes excess salts, also many have water storage tissues.
3. Some plants like the mangrove have pneumatophores( breeding roots) which emerge above water to obtain atmospheric oxygen from the air for respiration.
4. Mangroves growing on mudflats have buttress roots for support and Anchorage.
5. Most submerged halophytes are found close to the surface where there is sufficient light energy for photosynthesis. The deeper waters are capable of photosynthesising in low light intensities
6. The reproductive adaptations include floating fruits. The fruits have got large aerenchymatous tissues for air storage that makes them buoyant.
7. Water Lily produce seeds whose seed coat trap air bubbles make the seeds float on water which then carriers it away from the parent plant.
8. The fruit pericarp and seed testa are water proof. This way, the fruits and seeds remain afloat without soaking and sinking immediately they are released from the parent plant.