From Business Insider; mostly unappetizing. Pictured here is the 17th century watermelon, as cropped from Giovanni Stanchi's c. 1650s painting.
They look rough, but would have tasted great.
The watermelon originally came from Africa, but after domestication it thrived in hot climates in the Middle East and southern Europe. It probably became common in European gardens and markets around 1600. Old watermelons, like the one in Stanchi's picture, likely tasted pretty good — Nienhuis thinks the sugar content would have been reasonably high, since the melons were eaten fresh and occasionally fermented into wine. But they still looked a lot different.