I haven't been around in a while, but this question piqued my interest.

I will discuss grain tributes first, then briefly compare it to tax collection.

The Roman grain tax system was largely oriented toward the provinces of Sicily, Africa and Egypt, which were - together - the "grain basket" of the empire. The anonna was the name given to the system of grain movement from the periphery of the empire to the center, which enabled the city of Rome to reach (by many estimates) over 1,000,000 population. The system also, later on, encompassed Constantinople. It was a major blow to the Roman Empire's stable food supply when Egypt and then Africa were annexed by Omar.

The Roman merchant marine consisted of vast numbers heavy, cumbersome vessels, the smallest of which could move roughly 70 tons of grain in one shipment. Modern academic estimates (e.g. Charles and Ryan in "The Roman Empire and the Grain Fleets: Contracting Out Public Services in Antiquity", which I just found) state that some 150,000-300,000 tons of grain were moved to Rome per year. Simple math thus yields the quantity of grain which could have been sent to the city for the anonna.

Gaius of Roman legal fame writes that exemptions from public services and duties were granted to individuals who built ships with a carrying capacity of at least 10,000 modii of grain, so this was a benchmark vessel size (in today's terms: If you build a Panamax to move oil from the Gulf to California, you get a tax break).

The largest commercial vessels had three masts and relied on favorable wind. They were NOT oar propelled, only warships were. Maintaining slaves sufficient to propel such a ship would have been uneconomical.

Quite interesting in the legal perspective is the fenus nauticum, the maritime loan, which allowed interest in unlimited amount (vs the interest rate restrictions on other loans) because of the riskiness of maritime commerce. However, interest on interest was not permitted. In exchange for this punishing interest rate, the risk of loss was incumbent exclusively upon the creditor, not the debtor, liability was linked to the goods. Thus, if the goods sunk, the debtor was free of the loan obligation (see Mod. 22, 2, 1 et 3 CIC). Risk here was largely in the form of maritime disasters, e.g. rough seas, with piracy playing a subordinate role during most periods of the empire, given Roman naval supremacy. This is very century-dependent, however, and I am only giving a general overview. With the decline of the empire, and during other periods such as the early republic, piracy increased (especially from Cilicia during the Ciceronian period, if I recall). Celebreth will be able to give you more military history details.

In early imperial administration a prefect was instituted to govern the annona, the aptly named praefectus annonae. Ostia was the main harbor. Part of the reason for Ostia's construction was - as is common today in maritime infrastructure - to allow larger ships to dock directly and not need to offload at less opportune locations. Simple reason: larger ships have deeper draft, so they need deeper docks.

Regarding tax collection, roughly speaking, rights to individual provinces were auctioned off to interested publicans (tax collectors). Whoever bid the highest obtained the right to gather the revenue of a particular province, e.g. Egypt, plus a risk premium. The entire Roman tax collection infrastructure was outsourced to private contractors, in other words. The Roman government received a tax advance at the beginning of the year and was able to spend it prior to collection, whilst the publicans had the remainder of the year to recuperate their expense. This can be regarded as offloading the risk of tax default onto private parties and also as a way for the Roman state to debt finance itself to a small extent (the risk premium) whilst at the same time avoiding a massive, expensive bureaucracy.

So Roman grain and tax collection are best understood as a networked system of outsourced contractual arrangements and a quasi-public bureaucracy in tune with modern concepts of the "public-private partnership". It was understood in this way at the time and it is valuable to recognize that Roman thinking in terms of contracts, public administration and legal abstraction were at least on par with our own; the Romans were quite aware of the sort of structure they were creating and the ways in which it differed from purely public and purely private systems of tax and grain collection and distribution, as they discussed such topics in their legal textbooks (e.g. "Ius publicum est quod ad statum rei publicae spectat, privatum autem quod ad singulorum utilitatem," Ulp. D 1.4.4.2 CIC).

If you have any questions let me know. Roman law and maritime law are my specialties.