And for a serious answer...it doesn't matter that he has beaten the Hulk or Thing. Those guys are brute force. And while the Silver Surfer can go toe to toe with the Hulk in strength, his power cosmic puts him on a totally different level. He could vaporize the water in a huge bubble around them and take away Namor's advantage. He could carry Namor into space with him and see how the prince swims there. Or he could just blast him with the power cosmic, and use a ray with the power of suns. If he really wanted to he could probably turn Namor into a regular human.