The basis of implied warranty as to safe port or berth is that when it is for the charterer to nominate the loading or discharging place it can be reasonably expected by the parties that the charterer shall ascertain the safety of such place before nominating it because the owner would not know what port or berth the charterer is going to nominate.
Per Devlin J in Compania Naviera Maropan S/A v Bowaters Lloyd and Pulp and Paper Mills Ld (The Stork)  2 QB 68 at p.72:
There must be an obligation, express or implied, to nominate a loading place of some sort; if the ship is not told where to load she cannot earn her freight, and so, by failing to nominate or by nominating an obviously unsafe place the charterers would defeat the object of the charterparty without being liable for damages. There must therefore be an obligation to nominate at least one loading place, and there must be implicit in that some condition about safety to prevent the making of a derisory nomination. The obligation on the ship to proceed to a loading place "or so near thereunto as she may safely get" suggests strongly that the loading place itself must be safe. And when one finds the obligation on the charterers to nominate a loading place of some sort amplified by an express right to nominate one or two safe loading places, it does not require much effort of construction to conclude that the loading place which must be nominated must be safe.
Atkins International HA v Islamic Republic of Iran Shipping Lines (The APJ Priti),  2 Lloyd’s Rep 37 per Bingham LJ at p.42:
I accept (but subject to two important qualifications) that a safe berth is one which, during the relevant period of time, the vessel can reach, remain at and depart from without, in the absence of some abnormal occurrence, being exposed to danger which cannot be avoided by good navigation and seamanship. That seems to me to be an appropriate application to berths of Lord Justice Sellers’ famous statement, made with reference to ports, in The Eastern City,  2 Lloyd’s Rep. 127 at p. 131[see below]. My first qualification is this. Since, on the construction I prefer, the charterers had not promised that the port they declared would be safe, I do not accept that the vessel’s passage to and from a nominated berth should be treated as including any part of the voyage to or from the port. It would only include movement within the port to and from a nominated berth.
The second qualification is equally important. The charterers’ promise should, in my view, be understood as limited to a promise that the berth or berths nominated would be prospectively safe from risks not affecting the port as a whole or all the berths in it. To hold otherwise is to erode what I think is intended to be a meaningful distinction between berths and ports.
Mediterranean Salvage & Towage Ltd v Seamar Trading & Commerce Inc (The Reborn),  EWCA Civ 531, per Sir Anthony Clarke MR at para 33:
In the case where there is an express warranty of safety of the port but not the berth but the charterers are left to nominate the berth, the authors of Voyage Charters say at paragraph 5.42 that, if the port to be nominated must be safe, it follows that the berth must impliedly be safe. I would accept that proposition on the footing that it follows from Sellers LJ’s description of a safe port in Leeds Shipping Co Ltd v Société Française Bunge (The Eastern City),  2 Lloyd’s Rep 127. He said at page 133:
If it were said that a port will not be safe unless, in the relevant period of time, the particular ship can reach it, use it and return from it without, in the absence of some abnormal occurrence, being exposed to danger which cannot be avoided by good navigation and seamanship, it would probably meet all circumstances as a broad statement of the law.
per Rix LJ at para 62:
62. In sum, there is no authority which extends any implied warranty of safety to a voyage charterer’s choice of berth in a port which is not itself warranted safe. It appears to be accepted that a warranty of safety as to a port will encompass a warranty of safety as to its berths (Voyage Charters at para 5.42, Time Charters, 6th ed, 2008, at para 10.33). It seems to me that the corollary also applies: where there is no warranty of safety as to the port, there is unlikely to be any warranty of safety as to its berths, in the absence of an express warranty.