Today we have a tendency to think of underway replenishment and rearming as an everyday thing, but it was virtually unheard of prior to WW2 (in WW1 some "at sea" re-coaling was done, but it was a long laborious process, avoided at all costs unless absolutely necessary) and only developed later in the war out of necessity. Even today there are very few navies that have the equipment and capability to undertake this sort of rather complicated and dangerous maneuver. The game is much more forgiving than real life when it comes to this sort of thing, especially refueling where it can be done from merchant ships, etc., while in harbor. A very messy and time consuming process. In terms of reload cost and capabilities of ports, look at the port's "re-arm" number and that will tell you what weapons can be rearmed (look at the weapon load cost) by that port..
Further to dr.hal's comments.
Prior to WWI, one of the major considerations for extending the British flag across the world was to secure port coaling stations to enable the transit of RN ships from A to B, let alone conduct any operations away from home waters. Most coaling at sea was undertaken by German raiders who being denied port coaling facilities, relied on either rendezvousing with German coalers or capturing enemy coalers. It would take a day to refill the coal bunkers of the raiders. In fact the destruction of the German East Asian squadron at the Falklands can be attributed to the necessity of finding coal limiting the route options, thus enabling the RN to intercept it.