Go see if your local library has Chappell's book on American Small Craft, or his book on Boats; both will get slightly (tediously, occasionally) technical; but the information on the whys & wherefores is basic, and sound. I'm drawing a bit of blank on the other tome I was jsut thinking of that gives soem of the basic fundamentals of bouyancy, stability, and design (I hate not quite remembering things). Now, acopy of Knight's Modern Seamanship, or the Bluejacket's Manual, especialy from the 40s or 50s might be handily available from your local hal-price or used book vendor. Each discusses some the technical issues you are looking for.
Now, the classic (at least model-form) lobster boat is a "flat iron", "V" bottom form. The "ftat iren" name comes from the plan view looking like an old-fashioned flat iron. The hull bottom forms a variable taper "v" shape with a hard chine where the hull side meets the hull bottom proper.
Now, "floats like it's supposed to," is a bit of a dance. Salt water weighs (displaces) a bit more than 62 pounds per cubic foot; fresh water is right about 65#/cf. If we had a box of 1 x 1 x 3, and submerged half of it in water, it would displace 0.5 x 1 x 3, or 1.5 cubic feet, around 90-96# depending on water type. If the box only weighs, say 1 pound, it would take another 90# to get it to float at it's 6" "wateline." If the box weighs 20#, it takes less to get it down to that waterline. Alternately, the 20 pound box would "draw" only 2-3".
If we "know" from out prototype what the expected draft is, we can calculate the displacement. Foe flat iron hull, that's going to be somewhere around 0.48 to 0.55 of the lwl x wlb x d. If we use 1 x 3 x 0.12' is 0.36cf, x 0.5 is 0.18 x 60# is about 10#. Weigh your model, if it's 8.5#, you'd need 1.5# to balast it "right." Now, in practice, lighter can be better in RC. And weight distribution is often more critical than weight for displacement (that motorcyle battery might wind up too far forward to keep the prop in the water, which will limit performance no end).
Now, I used 'dance' up there to start, and on purpose. Because if we lighten the hull, as pointed out in another post, we "gain" displacement. If we have heavy items in the hull, we "lose" some too. Take from Peter, give to Paul and vice versa. Additionally, there's more than two dimensions at play. Ideally, we want the CB (Center of Bouyancy) to be under the CG (center of gravity). Pitch and roll prevent that from being static conditions. Certain hull configurations cause the CB to effectively "move" with heel (and pitch) in a way that creates a moment arm "back" to the CG. Where that moment arm "connects" to the CG is called the metacenter (MC, usually on plans).
From the hull section/body plan, this is simple, wide and low works nicely--and gives an ample MC height. Except, remember, the MC exists longitudinally, too. That's why many capital ships become very fine fore and aft in hull form while being quite beamy--they need to to decrease fore and aft pitch forces (lest one have a "bucking" gun platform). Back to take one from here, add one there.
So, sure, a person can whittle out a flat iron shape that will make into a fine lobstah baht, can I tell you what rise to give the floors up to the chines? Nope, not without a lot more info. Should a person add flim or flam to the frames above the floors? Dunno, H I Chappell goes into some detail on the virtues of one over the other. Strait frames seem much easier to-scale aft of midships, but you have to have some flare in the forward frames, or it just won't look right (or, it might just look like some home built boats the down'easters actually use <g>).
The Midwest kit makes for a better "jumping" off place in many ways. Not a bad idea, in my book to get a smaller, static version, and buid that up first, to get a feel for the shape of it, too. But, that's me, others differ.