I'm In Dutch St Maarten currently, and have swapped sides (and gone on out to Anguilla and back) in the last couple of weeks. Each time clearing in/out with the appropriate customs... I'm not sure if anyone would notice if you didn't, but it's easier on the conscience if you just do it properly.
Firstly the Dutch have recently doubled all their charges for entering and staying in the lagoon (or in Simpson Bay) so any cruising guide you have may be out of date. Of course you don't HAVE to stay in the simpson bay, or the lagoon. If you do enter the lagoon. There are two entrances to the lagoon (one on dutch side, one on french side) - both require bridge lifting, and the bridge(s) only operate 3 times a day in the high season - Dutch side charge a fee if you use the bridge (Collected when/if you clear customs), French side does not. If you are just passing through, and not stopping on the French side you can go through the French bridge and not clear in to French side - at least that is what Dutch Port Authority told us).
The French do not charge to be on their side in the lagoon, dutch do - so if you are trying to avoid costs, you might enter through the dutch bridge, and then go and check in in French side in Marigot, and anchor on the French side too.
Marigot Customs / Port Authority are CLOSED on the weekend, and close at 4pm other days. (We cleared in for a $5US fee) in Radisson Marina at Anse Marcel - over the weekend on route back from Anguilla.
Dutch side Customs/Port Authority is open every day until 5 (our cruising guide told us 3pm, but it was definitely later on a weekend even).
Dutch Simpson Bay Lagoon charges:
Need to pay from Bridge, lagoon, harbour fees as relevant and then a clearance fee also - see Fees & Info
Bridge fees during regular opening hours entering the Lagoon:
USD 10 for boats from 9 to 12 meters in length
USD 30 for boats from 12 to 15 meters in length
USD 60 for boats from 15 to 18 meters in length
USD 120 for boats from 18 to 22 meters in length
USD 200 for boats from 22 to 28 meters in length
USD 300 for boats from 28 to 36 meters in length
USD 500 for boats from 36 meters and larger.
USD 20 per week or a portion of a week for boats from 8 to 13 meters in length
USD 40 per week or a portion of a week for boats from 13 to 18 meters in length
USD 60 per week or a portion of a week for boats from 18 to 23 meters in length
USD 90 per week or a portion of a week for boats from 23 to 28 meters in length
USD 120 per week or a portion of a week for boats from 28 to 33 meters in length
USD 150 per week or a portion of a week for boats from 33 to 38 meters in length
USD 180 per week or a portion of a week for boats from 38 to 43 meters in length
USD 210 per week or a portion of a week for boats from 43 to 50 meters in length
USD 250 per week or a portion of a week for boats from 50 to 75 meters in length
USD 290 per week or a portion of a week for boats from 75 meters or larger
Customs Clearance fees:
USD 2 for boats of 1 – 99 gross tons
USD 5 for boats of 100 – 499 gross tons
USD 9 for boats of more than 500 gross tons
Harbor Clearance fees:
USD 5 for boats of 1 – 99 gross tons
USD 10 for boats of 100 – 499 gross tons
USD 20 for boats of more then 500 gross tons
French - MARIGOT BAY charges
Clearance fee: 5 euros
Initial administrative fee includes first day anchoring charges:
8 to 13 meters: 20 euros
13 to 18 meters: 30 euros
18 to 23 meters: 40 euros
23 to 28 meters: 50 euros
28 to 33 meters: 60 euros
33 to 38 meters: 70 euros
38 to 43 meters: 100 euros
43 to 50 meters: 120 euros
50 to 75 meters: 130 euros
more than 75 meters: 150 euros
Days 2 & 3: 0.25 euros per meter per day
Days 4 and thereafter: 0.35 euros per meter per day
(they might change this to all days at 0.25 euros/m/day)
Residents of French St. Martin:
0.13 euros per meter per day
No minimum fee. A 10m boat is 20 euros, then 2.50 euros/day for days 2 & 3.
No charge for French lagoon, Grand Case, etc... Only for Marigot Bay.
Based on honor system, but next week they will have a harbour patrol boat to check who is in the bay.