The boat was low to the water. - What does it mean? Does it mean that only a small part of the boat was under water? Or does it mean that the boat deck was near the water level? Or something else? Thanks a lot for your help.
I can see why you'd be confused here.
The boat being low to the water would look like this (excuse the poor drawing - this is the extent of my drawing abilities):
This, in contrast, is a boat that is not low to the water:
The boat that is "low to the water" has its upper rim close to the level of the water. In other words, it has sunken in more. This could happen if the load of the boat is heavier, for example.
"Low in the water" is a much more common choice, but "low to the water" seems to be a possible options as well. I would prefer "low in the water," as "low to the water" isn't quite as common as the other usage, especially in regular conversation.
SUPPLEMENTAL TO ALEX K's answer (upvote his answer, not this one!)
I'm not an expert on maritime usage, my knowledge being mostly confined to Hornblower and Aubrey novels. But I would have said that although both expressions versions express what Alex K describes, a boat whose height above the water is unusually small —
low in the water is what linguists call a "stage-level predicate": a temporary attribute of the boat. It describes a vessel whose freeboard (the height of the hull) above the water is lower than usual, presumably because the vessel is heavily laden.
low to the water is an "individual-level predicate", a permanent attribute of the boat. It describes a vessel whose freeboard and superstructure are less elevated than those of other vessels of the same sort.