In order to find the flotation plane accurately, you need an accurate model of the shape of the hull (which presumably you have in Rhino/Orca3D), and an accurate estimate of the weight and center of gravity, which means doing a weight study. This could be in Orca3D or in Excel (there are even dedicated software packages like ShipWeight, which are database applications for estimating and tracking the weight of a ship; in the case of an aircraft carrier, they might be tracking over 1 million weight items).

The advantage of the spreadsheet approach is that you can just start listing all of the items on the boat, without having to model them. You might have separate groups for the hull structure (hull skin, stringers, liner, seat foundations, etc.), tanks, electrical, plumbing, engine (the motor itself, steering, fuel lines, etc.), payload (passengers, wakeboards, cooler, etc.), outfitting (arch, cleats, etc.), etc.. In the end you could have hundreds of items. Some of them are very easy (e.g., you know the weight of the bow cleat and where it's located), and some are more difficult (the hull skin).

In Orca3D, items like the hull skin are relatively easy. You just create a material with the correct weight per square foot, and assign it to the hull surface (you may need to split the hull surface where the layup schedule changes). The software will then easily compute the weight and center of gravity. But for the bow cleat (and many other items that aren't explicitly modeled) you should create a "cleat" as a point object in the Material library and then put them in the correct locations.

There is a hybrid approach; use Orca3D to compute the weight and CG of the geometry that is explicitly modeled (like the hull), but use the spreadsheet for the final calculation (entering the hull weight and CG from Orca3D).

Then, when it comes time to compute the flotation plane in Orca3D, just enter the total weight and CG from the spreadsheet.