In your blender, puree the watermelon and lemon juice until you have a nice, smooth puree.
Divide the watermelon puree between 2 pint glasses full of ice, and top with the beer. There is a little bit of finesse required here, because pouring beer over ice can make a lot of foam. Tilt your glass, and very slowly pour the beer down the tilted side, stopping to stir gently if it starts to foam up. This is more a trickle of beer than a pour. Take your time pouring, and you'll end up saving time that you would have spent waiting for half a pint of foam to settle down. Continue tilting and slowly pouring until you have a full glass. Stir to combine, garnish with a lemon round, and serve!