Electrical Mystery

[ad_astra]

So I recently became the owner of a 1975 Tartan 27 in pretty decent shape. The boat did fine on the test sail and passed the electrical portion of the survey with flying colors. The test sail was two weeks ago and the boat was last run ten days ago, and has been plugged into shore power since then. Ten days ago the engine was serviced just before it last sailed.

Today I came to see the boat for the first time as the owner and found a substantial amount of water in the bilge (well more than normal) and also discovered that the manual pump does not appear to be functioning. After some exercise involving a portable bailing pump and a bucket, the bilge is now at a much more reasonable level.

The house battery has a whopping charge of millivolts, and the starter battery has a charge of 2.6 volts, so the reason the bilge hasn't been pumped is clearly electrical. Everything was turned off when I arrived..... What could possibly cause two working batteries to die in 10 days? Also, I'm pretty sure I know the answer to this question, but at that level of discharge, they need to be replaced for sure, right?