In addition to the vibration/shock/pounding that you mention, which is a very real concern in a small boat... You'll have to upsize the battery cable (lower gauge). There's calculations that can be performed (I don't know them off-hand, but Google can probably help) to determine how large you need to go based on how far away -- it's probably going to be something like 30' (most calculations will ask you for round trip - 60'). Now, price out 60' of large gauge, marine (tinned) battery cable and you're probably looking at somewhere between $200-$400, depending on size.
Secondly, batteries need to be vented because of the sulphuric gas that is given off.
Personally... my suggestion? Trim tabs are designed to be used - use 'em. I'm not sure on your boat, but Bennett trim tabs do offer an auto tab controller add-on that can help make it easier if you're not adept at using the tabs.
A better, and more productive, way to add weight in the bow is add chain to your rode.