This depends on multiple factors

Breast implants can feel very natural, or can feel like a "bag of water". This depends on multiple factors. First of all, Silicone implants absolutely feel much more natural than Saline implants. This is a fact. Secondly, the larger a woman's breasts are to begin with, the less palpable the implant will be after surgery. And third, if an implant is placed under the muscle, there is one more layer of tissue covering the implant, essentially making it more hidden and less palpable. Women who want the most natural feel to their breast chose Silicone over Saline, however, this usually requires a larger scar.  

Breast implants generally feel quite natural and real.  Some patients like the feel of silicone over saline but that varies considerably from patient to patient. Hardness of the breast implants is usually of capsular contracture.  With any sign of hardness, you should return to surgeon for evaluation.