You should find the volume of your objects and then do the necessary arithmetic which takes account of the water into which you are diving to fine the buoyancy.
If the objects sinks that is relatively easy because all you need to do is fill a bath with water and weight the object (luggage scales) when in air and when immersed in water.
The difference in the two readings in $kg$ can then be converted into a volume by noting that a difference of $1000 \, \rm kg$ represents a volume of $1 \, m^3$.
You could use a more accurate value for the density of water by measuring the temperature of the water in the bath and looking the density up in the appropriate text.
If the object floats then you can add weight(s) to just sink the sink the object but if this is difficult then add a weight which sinks the object and then use the luggage scales to find find the volume of the object and the added weight and then repeat the process with the added weight alone to find its volume.
The upthrust (buoyancy) is equal to the weight of water displaced and I do not think you need to make any correction for using mass rather than weight in your calculations.
The salinity and the temperature of the water at your diving location should possibly be taken into account?
Update as a result of a comment.
For an object which sinks:
scale reading with object in air (kg) - scale reading with object in water (kg) = mass of displaced water (kg)
$\dfrac{\text{mass of displaced water (kg)}}{1000} = \text{volume of object }(\rm m^3)$
assuming density of water is $1000 \, \rm kg\, m^{-3}$
For an object which floats.
Find the volume of an object which floats:
Use the above procedure to find the volume of the weight used as the "sinker" alone and then the volume of the object and the "sinker" but make sure that the object and the sinker are totally immersed in the water.
There are a lot of experiments to find densities using this method eg here and you are using a known density to find a volume.