Ocean water has a lot of salt in it which gives it a salty taste. Drinking a lot of it can make you sick, though, so the salt has to be taken out. Ground water has some salt in it but not enough to make you sick. Your town probably doesn't have enough ground water to for everyone to drink so they need to purify ocean water.

Interesting question...
Certain chemicals are more strongly bonded to chemicals in soils.This is because they have charges--a positive charge or a negative charge in the soils and the chemicals. Therefore, chemicals usually move very slowly. Since there is water in the ground, this water becomes contaminated. The contaminated water helps certain chemicals to move about and certainly some of which flows into the sea, but since the sea is so large...the result is a general "dilution of the pollution"--which seems to be the solution. So it is not that it isn't in the sea, but rather that it isn't in enough concentration in the sea to do much harm. Though over time this may change--specifically in localized regions. This means if all the water entering the sea in a specific location is heavily polluted or over a long time the sea becomes heavily polluted in one spot, then you will have the same problem with your drinking water, if you chose this place to build your desalinization plant. Note: some chemicals are heavy or don't breakdown very much under saline conditions and some will be disperesed or will break down. Once again it depends.
Good Question.

Living in Santa Barbara, we had a similar situation (actually the whole state was in a severe drought) where we were very low on drinking water reserves. Since at that time Santa Barbara did not receive "state water" - water redirected from northern California to southern California via the California aqueduct- we were desperate. Guadalupe was in a very similar situation. In both cases the decision was made to build desalinization plants (de=remove, saline=salts) which would take the salts out of ocean
water and convert it into fresh drinking water. The problem is desalinization of ocean water is very difficult and thus expensive. So it is really a last resort. Therefore, desalinization facilities are only used in the most dire of straits, when there is no other solution. The Santa Barbara desalinization plant hasn't been used since around 1989-90 when the drought ended (and it began to rain) because now our reserves fill up enough in the winter and spring to supply the county with enough drinking water for the rest of the year. Basically, its just a lot easier and cheaper to use rain water/ground water instead of ocean water.

I don't know for sure. That's a good question. It probably depends on where you live. The salts in the ocean come from the ground, either by flowing groundwater or in rivers, so the things in the water in the ground should be pretty much the same as the things in the ocean, except that there will be less of them. On the other hand, ocean water is pretty much the same everywhere because it gets mixed by waves and ocean currents, but ground water will have things in it that have to do with the rock it is in. So there could be more of some things and less of others. Also, I don't know how much gets filtered out in the making of drinking water from ocean water. It's a good question.