Water in Basement

That sounds as if the house doesn't have a "real" basement (made with the basement tool), but just a normal story built downwards. Those flood if they lie below the water table of the ocean.
Given the basement function was only added later to the game, there are many houses for download that have "old-style" basements.

@kaitlyn0 Those "basements" that are built via the old method (by lowering the ground with staircases) are also fully underground, so that's not a valid criterion to distinguish them from those basements built with the basement tool.

Anyway, if your basement is flooded, it's one built with the old method, and you probably put the lot somewhere on the beach. Either relocate the lot somewhere else or replace the basement with a real one. If your house has a foundation, you will still need the old method to get to the base of the foundation and can then use the basement tool from that point on. "Real" basements can be several stories deep without flooding.

Yes - you need to place that lot on higher ground so it is above the top of the water table.

I often place basements using the basement tool on the beach and have them 4 levels deep with no water coming into them. But I've had others tell me they do get water in those basements too. I have noticed my sims splashing along as they walk sometimes when in one of those basements on the beach but there is no water visible. The sound effect when walking sounds like they are splashing in shallow water. I tend to ignore it if that happens - but it doesn't happen often.

"Karritz;c-17429999" wrote:
Yes - you need to place that lot on higher ground so it is above the top of the water table.

...

As @Karritz says it needs to be on higher ground. You can

1. pick a different lot that is already on higher ground
2. or try to place it on the lot you already chose by using "SetImportedTerrainOffset positive number". It will place the lot on a plateau. Hopefully you can use terrain smoothing tools to make it look more natural. You will probably need to experiment with the number to get it high enough so there is no water in the basement but not so high it looks weird. Be sure to use "SetImportedTerrainOffset 0" when you are finished so that every lot you add to this town is not placed on the same kind of plateau. The smoothing out works better if the lot where you are placing the house is larger than the one being placed.

"Turjan;c-17429144" wrote:
That sounds as if the house doesn't have a "real" basement (made with the basement tool), but just a normal story built downwards. Those flood if they lie below the water table of the ocean.
Given the basement function was only added later to the game, there are many houses for download that have "old-style" basements.

I'm not a builder so I'm not 100% sure, but I think it is a "real" basement because half of it is fully underground.