Updated
Citizen Promaster Sea Eco-Drive Diver vs Casio G-Shock GA-B2100 CasiOak
Two divers compared — $250–$500 vs $100–$150
Quick Verdict
The Casio G-Shock GA-B2100 CasiOak ($100–$150) delivers Casio Quartz (Solar-Powered) technology in the tier below the Citizen Promaster Sea Eco-Drive Diver ($250–$500). The G-Shock GA-B2100 CasiOak measures 45.4mm (48.5mm lug-to-lug, 11.9mm thick) and is rated 200m (20 ATM). The Promaster Sea Eco-Drive Diver justifies its premium with Made in Japan heritage and Eco-Drive Caliber E168. Both represent excellent choices in the diver category.
Transparency — We earn a small commission on purchases made through our Amazon links. This doesn't affect our editorial independence.
Specifications Compared
| Feature | Citizen Promaster Sea Eco-Drive Diver | Casio G-Shock GA-B2100 CasiOak |
|---|---|---|
| Price | $250–$500 | $100–$150 |
| Case Size | 44mm | 45.4mm (48.5mm lug-to-lug, 11.9mm thick) |
| Movement | Eco-Drive Caliber E168 | Quartz Module 5689 (Tough Solar + Bluetooth) |
| Caliber Type | Citizen Eco-Drive (Solar Quartz) | Casio Quartz (Solar-Powered) |
| Power Reserve | — | About 7 months on a full charge without light; about 18 months in power-saving (Casio) |
| Water Resistance | 200m (20 ATM) ISO 6425 | 200m (20 ATM) |
| Crystal | Mineral Crystal | Mineral Glass |
| Case Material | Stainless Steel | Carbon Core Guard Resin |
| Origin | Made in Japan | Made in Japan |
Category-by-Category Analysis
Design & Aesthetics
Citizen's design language vs Casio's approach
The Citizen Promaster Sea Eco-Drive Diver features a Stainless Steel case at 44mm with Mineral Crystal protection. The Casio G-Shock GA-B2100 CasiOak offers Carbon Core Guard Resin construction at 45.4mm (48.5mm lug-to-lug, 11.9mm thick) with Mineral Glass. Both represent their brand's distinct design philosophy and heritage.
Movement & Performance
Eco-Drive Caliber E168 vs Quartz Module 5689 (Tough Solar + Bluetooth)
The Promaster Sea Eco-Drive Diver runs the Eco-Drive Caliber E168 (Citizen Eco-Drive (Solar Quartz), standard power reserve) at standard frequency. The G-Shock GA-B2100 CasiOak uses the Quartz Module 5689 (Tough Solar + Bluetooth) (Casio Quartz (Solar-Powered), About 7 months on a full charge without light; about 18 months in power-saving (Casio) reserve) at standard. The G-Shock GA-B2100 CasiOak offers a longer power reserve.
Wearability & Fit
44mm vs 45.4mm (48.5mm lug-to-lug, 11.9mm thick) — different wrist presence
At 44mm, the Promaster Sea Eco-Drive Diver wears more compactly on smaller wrists compared to the G-Shock GA-B2100 CasiOak at 45.4mm (48.5mm lug-to-lug, 11.9mm thick). Water resistance differs: 200m (20 ATM) ISO 6425 vs 200m (20 ATM).
Value & Cost of Ownership
Casio sits in the lower tier
The Citizen Promaster Sea Eco-Drive Diver sits at $250–$500 and the Casio G-Shock GA-B2100 CasiOak at $100–$150. On the specs that outlast any price: the Promaster Sea Eco-Drive Diver uses Mineral Crystal with a standard reserve, against Mineral Glass and About 7 months on a full charge without light; about 18 months in power-saving (Casio) on the G-Shock GA-B2100 CasiOak. Service intervals are typically 5-7 years for both.
Water Resistance
200m (20 ATM) ISO 6425 vs 200m (20 ATM)
The Promaster Sea Eco-Drive Diver is rated to 200m (20 ATM) ISO 6425 while the G-Shock GA-B2100 CasiOak handles 200m (20 ATM). Both offer identical water resistance ratings.
Our Verdict
Citizen
Promaster Sea Eco-Drive Diver
Choose the Citizen Promaster Sea Eco-Drive Diver if you prioritize Citizen Eco-Drive (Solar Quartz) engineering, Made in Japan craftsmanship, and 200m (20 ATM) ISO 6425 water resistance. At $250–$500, it delivers Eco-Drive Caliber E168 with solid power reserve.
Check Price on AmazonCasio
G-Shock GA-B2100 CasiOak
Choose the Casio G-Shock GA-B2100 CasiOak if you value Casio Quartz (Solar-Powered) technology, Made in Japan heritage, and 45.4mm (48.5mm lug-to-lug, 11.9mm thick) proportions. At $100–$150, the Quartz Module 5689 (Tough Solar + Bluetooth) with About 7 months on a full charge without light; about 18 months in power-saving (Casio) power reserve makes it a compelling choice.
Check Price on AmazonThe Bottom Line
The Citizen Promaster Sea Eco-Drive Diver and Casio G-Shock GA-B2100 CasiOak represent two distinct approaches to the diver category — the Citizen bringing Made in Japan tradition while Casio delivers Made in Japan engineering.
Frequently Asked Questions
More Comparisons
Explore These Watches
How This Page Was Made
Our comparisons are generated from a hand-maintained specification database and reviewed under our editorial standards. We use AI tools to assemble pages from that data. We don’t invent specs: every number traces back to the database, and when we correct an entry, the fix is applied site-wide. As an Amazon Associate, The Horologist earns from qualifying purchases.