A rental is just a non-event ship that you don't get to keep forever. They can be purchased or given to you (both temporarily). Those three ships were added to your account for free and you can use them (or not) for whatever you want for the rental period. Bear in mind the rental is 7 days and the personal mission is on for a month, if the rental period runs out you will not be able to complete the mission without getting those ships again somehow. 10k free XP and 3 regular pumpkin boxes (each being 3 decent camos or 5k free XP or a permanent halloween skin in rough order of rarity) is a decent reward for just getting victories in cheap to repair and decent earning potential premiums that you can use to train up any American commander. If you check the email account associated with WoWS you should have an email from the 11th (or that's when I got it at least) titled "Welcome Back to World of Warships". I believe this is one of those offers to entice players to come back to the game. I have been relatively active in the game recently (3 months), but only PvE; I suspect that the absence of PvP recent stats is what triggered me getting the mission, on the off chance you are in a similar boat.