The rock thing is a bit random. I mean, sure, lifting heavy **** can help you get stronger but with the rock and/or sandbag how can you progress? There's only so much room for sand in the bag so really you can only increase strength by increasing the number of reps you can do. Eventually this is more of an aerobic workout then strength building. Good for cutting fat but not for building muscle. So if getting stronger is your goal then you should really look into a gym membership or an area for that power rack so you can increase your weights and see real gains.
As well with a beach to workout with you would be better served doing sand sprints and open water swimming for fitness, fat loss and building muscle. Sand Sprints are incredibly difficult and build muscle strength, cardiovascular capacity and increase your overall badass level. Open water swimming is way more difficult then swimming in a pool and will help build muscular endurance in almost every muscle in your body. Doing these types of workouts help to increase strength in muscle fibers you just can't hit with lifting the heavy stuff. But when you go back to the lifts you will notice that your strength has increased because the muscle is getting a more complete workout.
I personally love working out on a beach. doing plyometric work and bodyweight exercises in the sand is challenging and increases your balance and coordination. Try doing 100 burpees in deep sand and you'll see what I mean.