The Wikipedia page for the P-3 says
Empty weight 35,000 kg
Loaded weight. 61,400 kg
Max T/O weight 64,400 kg
So yes, 26,400 kg is the difference between empty & loaded. It therefore includes fuel, crew & everything else, not only weapons. Capacity of bomb bay & external hardpoints is given as 20,000 lb, i.e. 9000 kg.
The P-8 empty & max T/O weights are given as 62,700 & 82,800 kg, but this is puzzling, as the empty weight of the basic 737-800 is only 41,400 kg. The P-8 might weigh more, because it's modified from a higher weight 737-800ERX (can't find empty weight), & it has extra equipment, but over 20 tons more, when max T/O weight is only 3 tons more? And all the passenger-specific stuff which wouldn't be carried (seats, lockers, food, drink, galleys etc.) has to be offset against the extra stuff.
The difference between empty & max T/O weights is less than the weight of a full fuel load for the basic 737-800, but the 737-800ERX should have more fuel capacity, & the P-8 at least as much. The Wikipedia P-8 empty weight isn't credible.
The US Navair site says the P-8's max T/O weight is 85,800 kg, but even that isn't enough extra for the Wikipedia empty weight to be credible.
Aha! The Naval Technology site says that the P-8 has extra fuel tanks in the hold allowing it to carry 34,100 kg of fuel.
I think it's probably best to accept the US Navair max weight. That gives 51,700 kg less max fuel. I'd guess that it has about the same weapons capacity as the P-3, & empty weight is only a few tons more than the 737-800.