Man, that slipping feeling is honestly the worst when youre trying to frame a shot. I would suggest being really careful with weight ratings since that 100-400mm is a beast. You might want to consider these budget-friendly options that punch above their weight:

• Benro Mach3 TMA28C Carbon Fiber Tripod
• Innorel RT80C Carbon Fiber Tripod Make sure to check if the locks feel gritty after the beach. Let me know if you need help picking a head!

Just saw this thread and honestly, I'm worried you might spend $400 and still have the same problem if we dont pinpoint the fail point. Before I give a full recommendation, can you clarify if you are mounting the setup via the camera body or using the tripod collar on the 100-400mm? That lens is long, and if you arent using the collar, the center of gravity is way off, which explains why the ball head keeps sagging. Coastal wind is a different beast entirely. You might want to consider these factors before buying:

• Load capacity specs are often marketing fluff. I would suggest getting a head rated for at least triple your gear weight for coastal work.
• Aluminum legs are sometimes better in high wind because the extra mass helps damp vibrations, while light carbon can vibrate like a tuning fork.
• Salt spray is a killer. Make sure to choose a model that is easy to disassemble and rinse. I would suggest looking at the Leofoto LS-324C Ranger Series Carbon Fiber Tripod with LH-40 Ball Head. It is very low profile which helps with stability. If you want something even beefier, the Vanguard Alta Pro 2+ 263AT Aluminum Tripod Legs paired with a Sirui K-30X 44mm Ball Head is a rock solid combo that should stay well under your budget. Just be careful with that center column on the Vanguard... always keep it down when the wind picks up.

To add to the point above: I honestly wouldnt trust those lighter setups for windy coastal shoots. I had issues with vibration on similar Benro legs before and it was pretty disappointing to see the soft images later. Unfortunately, most ball heads in this price range are not as good as expected when you tilt them with a heavy lens like yours. If you want something rock solid, you need a design without a center column. Center columns are basically just giant vibration magnets in the wind.

• Leofoto LS-364C Ranger Series Carbon Fiber Tripod
• It is much more stable because the legs connect directly to the apex.
• Sirui K-40X Ball Head 35kg Load Capacity
• This is one of the few heads under 200 bucks that actually locks down that 5D setup properly. Quick tip: Use the lens collar and balance the weight over the center. If you mount by the camera body, even a high-end head is gonna struggle with that heavy glass.