My H400 is soon 15 years old, and my keel bolts and plates look like new.

I have not heard of seawater under the floor, but some members have had seawater in the bow, under the anchor locker. This compartment is closed, but if it goes undiscovered for some years it may float to the main cabin. First time I emptied the water I removed 10-15 liters. The most likely reason for this seawater is ingress from the anchor lockers drain hole. Use the search function, StavrosNZ and others have written some posts about this.

Freshwater or diesel in the bottom of the boat have happened, use the search function.

If you state your country, and which seas you are sailing in, (and maybe a few more facts), it will be easier to answer your questions.

Ten years ago a Texan couple asked me for help regarding a lot of moisture running down the inside of the hull of their Amel ketch. They were sailing towards colder climates, and had never experienced condensed water running down the walls. I recommended a dehumidifier.

A sailor from Murmansk or Scandinavia would know instantly the cause for the condensation. Here at the North side of the globe, our houses are heavily insulated for that same reason.

In my opinion, the corrosion in your pictures is a cosmetic issue, not a problem.

The boat is seven years old, and at one time there has been enough moisture (salt or fresh) to corrode the surface of the plate. The galvanized bolts seems undamaged.

Use the issue to get a discount on the price.

:Kjeld

P.S.: The discolouring on the plates in your pictures looks more like wooden dust, sand and dirt than corrosion/rust to me.

the discolouring is def corrosion as i took the pictures myself. Agree though only the plates and not the bolts are affected. It is also sw related corrosion but of course the question is where it came from. 

Salt water can get in when removing the speed transducer for cleaning.  If this went unnoticed it might possibly result in this corrosion.

I've just been sailing on a boat with a deep sump over the keel bolts where 5-10 litres of water collected.  Due to the depth of this sump it was difficult to remove whatever was left after using the bilge pumps.  We tracked down the issue to a leak on the shower pump, but it had clearly been like that for some time.