Based on the level of respect or formality, you can say "I like watermelon" in Korean in 3 ways:

• Informal: na-neun subag-eul joahae (나는 수박을 좋아해)
• Formal: na-neun subag-eul joahaeyo (나는 수박을 좋아해요)
• Very formal: jeo-neun subag-eul joahamnida (저는 수박을 좋아합니다)