In v1.19 Mojang added boats with chests to the game that can be used to transport items along with a player or mob across bodies of water. This is a welcome addition to the game as previously, players had to make multiple trips across water bodies to transfer items from one location to another.
Players can craft a Boat with Chest quite easily, all you need is a boat and a chest. Combining the two is fairly simple, you will have to place the chest and the boat anywhere in the crafting grid since no special formation is required, and you have yourself a Boat with Chest.
The inventory size of the Boat with Chest is identical to that of a single chest. You can access its inventory by interacting with the boat while sneaking (crouching). The boat can carry only one entity with the chest whereas regular boats can carry two entities. The ability to transport more items with you while traveling over water bodies is useful to have.
The addition of the Boat with Chest has been a great and quite useful addition to the game. They even work with hoppers just like the Minecart with Chest, making them usable in redstone contraptions. It allows you to move large quantities of items over water bodies such as oceans and lakes.
A. To craft a Boat with Chest, you need a boat and a single chest.
A. Yes, Boat with Chest was added to Minecraft in the latest update v1.19
A. The inventory size is identical to that of a normally placed single chest.